The simplest, cheapest and fastest thing is to get a sheet of ND lighting gel and cut it to fit your filter holder. You don't have to go to a stage lighting store or anything, just order one from B&H, Amazon, etc. About $8 a sheet usually. Get a 1/2 ND, trim to fit, and you can stack as many sheets as you want. Treat and store them like your contrast filters. The sheets are about 18x24".
And the Ph11 the OP mentioned is a standard edison-base 110v frosted bulb, not a low-voltage light. It can be dimmed with a dimmer from the hardware store, or buy a "router speed control" from Harbor Freight/etc, but it will warm up the color temp of the bulb. I'd just use the ND and stop messing with "If I mix a 2.5 with #4..."
I did this all the time in the pre-film days, duping color slides to color sheet film with a cheap flash head taped into the condenser box to get daylight color (the lab said it was impossible to dupe onto Velvia... it wasn't.)