Ok, where to start.....
You're using soilless mix so your pH will drop as the medium dries out. pH to between 5.6-5.8 before feeding. It's best to have a slightly different pH range each time so your plant gets to take advantage of the full pH absorption range.
If you've added perlite to your mix, you can probably water till you get 10-15% runoff and have better results. Make sure your pots dry almost completely out before watering... you want the plants dry but not wilting. This setup will allow you to run a slightly higher nute concentration than straight pro-mix (without issues, that is.)
If you haven't added perlite you may want to give the plant only enough nutrient solution to last 24hrs before drying out. You don't want any runoff. In this situation you're going to need to use a weaker solution as the nutrients will build up in the medium over time.
No matter which solution you use you're not going to use full strength Lucas Formula since you'll burn the plants.
What you're seeing right now is a deficiency from your nutes not being strong enough. 2 week old plants under intense CFL or HID lighting can handle the Lucas Formula at full strength in hydro so don't be afraid to bump the nutes up a bit.
Don't need to pH the Lucas Formula when using R/O? Uhh.... that's some bogus info... please don't listen to that. If' you're LUCKY.... you may not have to adjust pH. I have always had to... for 10 years now and I've always used R/O water. *shrug*
You have 2 choices here:
1. Mix a gallon of full strength lucas and water your plants every 3rd watering with it. When your plants look a bit too well fed, add an extra day of watering. Make sure you don't use pH'd water on your plain watering days.... leave it straight R/O or you won't get the pH swing your plants require.
OR
2. Mix a gallon of 1/3rd strength lucas and water your plants every day with it.
Either way you're going to want your promix to dry out every 24hrs.
Stay Safe!