Well absolutely I'm going to confuse the guy with different advice...
*Don't* alter the mixing ratio. Have clean water available, as you would for any calf - you don't need to feed all their fluids via bottle and the calf doesn't need proportionally more fluids than a larger calf.
Although the bag never specifies it (that I've used) the recommendations are usually for a 90lb calf. Your calves are 2/3 the size and should start on 2/3 the milk, mixed as directed. It was 125g/litre when I was using milk powder, but may be different with other products.
Alhtough I think a lot of scours in small calves is directly related to rearers expecting them to drink the same quantity as much larger ones, it's also true that they tend to be less robust, more susceptible to problems... so take that bit extra care with feeding and bedding down and general hygiene and you should have a good result.