i know the method is outdated, thats why i stated rule of thumb idea...but you asked for it, so here it is.
125DT= 640w of VHO, 525w of MH (4x 72" 160w VHO + 3X 175w MH)
20L frag/nano= 150w of VHO, 70w MH (2x 24" 75w VHO + 1x 70w MH)
i will soon be upgrading the 20L to a 29....
future 29DT=...