>And SINGLE and DOUBLE don't work properly for the return value.
>

Actually, from a bit of experimentation with Christof's help, we proved out that SINGLE and DOUBLE put the value back into the variable, but it doesn't update the value with the new precision, resulting in some funny rounding. As I pointed out, pre-initing a DOUBLE variable to a value with correct precision seems to work OK (IOW, init the variable receiving the DOUBLE or SINGLE to 0.00000000000000000000) and it seems to work OK; it definitely fixed the problem with DOUBLE passed by reference and copied in and out using RtlMoveMemory() in a parameter.