Per lumen output, ie taking consideration that t-8's put out more light then the t-12's (WPG
rule is based on t-12 output), you are at 120 watts. So 3 WPG
with two 25 watt bulbs. That is plenty of light, but if you decide someday to increase it, just add one more bulb and you are at 4.5 WPG
FWIW, I was able to finish my DIY light strip. Here are some pics of how close I put the lights together. If you run them straight, no ODNO, you can fit them in pretty tight and get more light out em as Mr. F was saying. 2 bulbs would save you money of coarse, but 4 bulbs would put more light out. This is totally dependant on your setup of coarse. Just giving you options.
My old setup was two 25 watt CF
screw-ins and 15 watts of t-8. In the new setup 2 bulbs say 15 watts and 2 say 18 watts. Total wattage of 66 watts as compared to 65 on my old setup. I can tell you without a doubt that even though the bulbs say 15 or 18 watts on them, this fixture is putting out way more light then my previous setup. I don't know way the bulbs have different ratings on them, but once I put the new fixture on my aquarium, my plants pearled like they never have. ie they are getting more light even though the bulbs only say 1 watt more.