This is a late reply, I know.
I switched to TSOP4838s and I tried your equation on them.
R = 3
I = 5 (usable voltage)
A = 0.004 (usable current)
R/I/A = 150? That can't be right. That would mean I can use 220 of these things in parallel...
What is the value of R here? I carried it over from your equation but kept it the same because I wasn't sure what it represented.