You can put the light on 18/6 from day 1 [keep an eye on the temp high/low]
Start feeding after about a week, when the first pair of blades have appeared
Here is their feed calculator as a guide
Grow Guide | CANNA UK
Basically, young plants 1-2ml/L, vegging plants 3-4ml/L then start of flower 1 week of 5ml/L + PK, then reduce to 3-4ml/L through the rest of flower - the plant will tell you if it's hungry or stuffed by the colour of the leaves
Don't overdo the Doff or it can rot - just 1-2ml/L every 2nd or 3rd watering in veg
1L of water > 1ml CaMg > A>B > pH 5.8-6.2 > 1-2ml Seaweed [if using]
The oscillating fan should be on for circulation, and a slight breeze will strengthen the stem but don't aim the fan directly at the seedling or you'll get wind burn