Depending on where and how fast the water is, for slow pools on the Cap I'll go as light as 11,15 and 20. For the C/V I like to mostly use 25 and 30, 25 is my usual float of choice but sometimes it can make light bites hard to detect so making sure you balance out the float with weight is a good idea.
One thing you can always try is taking a long tube or a VERY tall glass and filling it with water and experimenting with float size to weight, cutting pieces of pencil lead to length, using various split shot and other types of weights till the float sits at the desired height, but remember that the type of presentation/ weight of bait/ ect at the end of your line can affect how your float behaves/ sits on the top of the water so always keep that in mind as well.