When adding UV make sure the bulb is giving you UVb. UVb during the last couple weeks of flower will help produce higher % of THC. During Vegg and early flower has no positive affects.
Higher levels of UVb creates stress. One of the ways the plant protects its reproductive areas is to produce more THC in the trichs to help reflect the harmfull rays away.
You are trying to mimic the intense light during peak hours of daylight, which have the most UVb levels. But DON'T over do it!!! The last two weeks of flower I give mine 4 total hours during the middle of the 12 hour cycle in 2 hour segments.
Or, for 12 hours, 3off+2on+2off+2on+3off.
Peace, Superman
PS> The bulbs I use are T5 HO with 75% UVb and 25% UVa. They came from HTG supply for around 28 bucks each.