So, your using the Lucas formula? Awesome! I would concur with the other posters who say raise it to 1200. Lucas himself states that the lucas formula should be used at 1300 PPM throughout the flowering cycle, up until flushing the last week.
Yes, going with the grow formula a couple weeks into flowering will cause some extra stretching but they will turn into nice long colas before too long.
Lucas states that anything between 1100 and 1400 PPM during bloom is adequate (when using the Lucas formula), with a target of 1300 being perfect.
I am using the lucas formula right now and I try to keep the PPM between 1200 ppm and 1300 ppm, and have done so since week 2. IF you are using the lucas formula at 1000 ppm, bring it up to 1300 and keep it there.
I have found that if I use half strength Lucas solution in my top off water, the PPM stays pretty close to perfect most of the time. My PH is also more stable. My PPM will drift to 1400ish between topping off, the half strength formula knocks it back down to about 1200.
You can get absolutely stunning results with the lucas formula and it has more calcium and magnesium than the 1-2-3 formula that GH recomends. This eliminates deficiencies. It also eliminates problems with nutrient lockout, etc.
I have found that the phosphorus is used a little faster than the nitrogen when using Lucas. I change out my resevior solution every 2 weeks. If I keep the solution right at full strength the whole time, during the last few days I get some tip burn from the nitrogen buildup. Because of this, the last 4 days or so, I top with two tablespoons of bloom in five gallons of water, and very little micro. This eliminates tip burn but they don't miss a beat.
by week three you should see some nice calyx formation with lots of white hairs popping out. Your plants will stretch a little more over time. You can calculate the time they will stretch. The stretching phase lasts about 40% of the budding cycle. A ten week flowering strain will stretch for 4 weeks. A seven week strain will stretch for about three weeks.
Bring up your ppm to 1300 and keep it there and let the magic happen!
Hope this helps.
Peace V