How do I derive an expected total for a subset of items based on the expected overall total?

You can derive expected totals for subsets of items by taking fractions of the expected total of all T items IF each subscale has the same expected value and each subscale is weighted equally in computing the total scale score. Expressed more formally

If each of T items has the same weight, W, and expected score, E(S), then it follows for 1 <= t <= T:

E(total of all T items) = TW E(S) and 
E(total of a subset of t items) = [TW/t] E(S) 
= E(total of all T items)/t

For example if t=2 then do obtain the expected total for half the items you just need to half the total for all T items.

If neither the weight or expected score is different for each item score then it is not possible to derive the expected total score for each subset of items based on the expected overall item total by simple fractionation of the overall item expected total (as above). In this case it is necessary to know the individual item weights and item expected scores to derive the expected total for each subset of items.