It all depends on how much heavier the FC brass is than the Win. I sure don't have an answer for you that's specific, but remember that brass is about 8 or 9 times more dense than powders are, so an 8 or 9 gr average difference if it's all in case thickness might be 1 gr difference in powder charge.
Without a chronograph the best thing to do would be to check case capacity with water or better yet methanol or denatured alcohol. Weigh a fired sized case with a primer (fired or not), fill it with methanol or denatured alcohol, reweigh it, and get the weight of alcohol. Methanol has a density of ~0.79 (water = 1.0), denatured is about 0.8 depending on the denaturant, so you can figure out the internal volume of the cases. The internal capacity of the cases will be about 10% more than the weight of alcohol which will give an idea of the actual difference in case capacity.
If you have some fine ball type powder it (TAC and X-Terminator for instance) it could be used too, and would be simpler and give a powder weight without calculation. Fill the cases to the mouth (trimmed to the same length preferably) and level then weigh.
