Ah, 38 TOTAL not 26, much better, perhaps a bit high, maybe not, certainly better than 26.
At what rpm did you get max mechanical 26 degrees advance?
Yes indeed the vacuum signal should be SO low (or virtually nonexistant) at WOT that it wouldn't be a factor. The other thing I might ask myself (just for curiosity) is at vacuum signal does the vac advance start to work? Often it takes 15" or so to get any movement of the vac advance.
Lets say your cruise rpm is 2800, full mech advance doesn't come in until 4500 rpm. Vac advance doesn't come in until 15" of vacuum signal strength. Your cruise vacuum is 12", below the threshold. The main reason to USE vacuum advance is to increase timing under cruise rpm for best gas mileage. So it is important to 'work the numbers' to see if in fact you can\are taking advantage of the vac advance under 'real world' conditions.