The CE Epd does not address weight. It is an attempt to compare the ease of calving between different bulls. Like Kenny points out, sometimes a heavier calf can be delivered easier than a light one, because of the shape of the calf. Even the birth weight Epd does not predict an actual calf weight. For example, if a herd of cows is now averaging 80 pound calves when they are bred to highly proven bulls with a birth weight Epd of +2, they should average 76 pounds if they are bred to a highly proven bull with an BW Epd of -2. The cow is 1/2 the equation. Actual birth weights may vary from 60 to 100 pounds, but average 80.
Your cows, location, and time of year you calve will all affect birth weights. Cows in a Northern cooler location will have heavier calves than cows in a warm Southern location. Calves born in the Fall will be lighter than calves born in the Spring because the cows were gestating those calves in warmer temperatures in Fall. If you have a cow herd that has averaged 85 pound calves when bred to a variety of different bulls, you will not find a bull of the same breed that will consistently give you 65 pound calves. The Dam has more influence on birth weight than does the sire. I am not sure what breed you are referring to when you say 8-10 for calving ease, but that will impact your results also. A 10 for a Simmental is not the same as a 10 in Black Angus. If you want 65 pound calves, breeding to a Jersey will be more likely to produce that result than if you breed to a Hereford.
I am not sure where you are located, but on my Ranch a bunch of 65 pound calves would be a reason for concern. I calve in cool temperatures and my cows can easily handle calves over 90 pounds. I would hate to give up 25 pounds at birth and hypothermia is a bigger threat for those little ones. If you are calving 1000 pound cows or are located in a warm climate, then 65 might be fine.
80-90 lb BW's are way to high for my cows. they aren't that big. they average around 1200 lb. some smaller. my bull now puts out around a 60-65 lb in a heifer and a 65-70 lb out of a cow. That's about where I'd like to stay. I don't like to pull calves.
I believe I have read somewhere that about 7% of the cows weight is a good birth weight. At 1200 pounds I think your cows could easily handle an 85 pound calf. My registered Angus cows are mostly about that same weight. I have never had a problem with a 100 pound calf in an adult cow, but I do look at the CED. I see you are in Kentucky. I expect your temperatures are warmer in the months prior to calving, so that is probably why your calves are so light. I would bet I could take at least 10 pounds off my birth weights by moving to your area so that we are not always calving in a freezing rain. I would love to move my calving dates, but it often dries up and the grass dies here by June, so I need to calve early enough to take advantage of grazing.
Do you actually lift and weigh your calves with an accurate scale? Most people underestimate birth weights and if you are using a tape to estimate weight, I found that tapes under-estimate the weight of my Angus calves. I expect this is because they are built for ease of calving at birth and are therefore longer and not as big around. I tried both the girth and the hoof tapes and neither were accurate. Both gave me lighter weights in many cases than the actual scale revealed.
I think you are much more worried about birth weight than you need to be. The only advantage to a 65 pound calve I have found is that they are easier to lift when you weigh them. My own experience is that the 65 pound calves are usually 50-100 pounds lighter at weaning than their 85-90 pound herd mates.
The way I look at it, if you never had a problem due to birthweight, don't strive for a lower one... I'd look at the CE more than the BW.. if you can add 10 lbs of birthweight and not increase birthing difficulty, you're ahead.
First off given your estimated cow size I would agree there shouldn't be a huge problem if your birthweight did increase. Granted, like you said, you aren't sure on their size. If you are comfortable at what your current bull throws for calves then why not go for a bull with a similar BW and CED EPD.
Also the BW EPD of the bull has less to do with your birthweights than other factors. Management has a lot to do with it. Cow size plays a role as well. The cow's BW EPD plays in as much as the bull's. When you know what those cows give you with a known EPD then you can have an idea of what introducing a different bull with a different EPD profile would give you