in general since chlorophyll a and b both got an absoption maximum in blue and red wavelengths you could always use VEG and BLOOM.
But the light you are using has only one intensity when turned on in either modes so i would do it like
@PlatinumLEDTom and
@namvet25 said,
turn it on when u see first signs of preflower but start with more distance to the plants because the added intensity/heat/energy could burn your plants in their early stadium.
For more details i took some info about the P900 from the website:
Veg Power: ~280W
Bloom Power: ~520W
so if you turn on your Bloom you triple the output of the light. This means you have to get more distance to the plants.
In general the law is, if you double the distance to the plants the power will be 1/4.
so in an example:
Distance: 0,5m = power 1
Distance: 1,0m = power 1/4
So to be sure not to burn the plants i would multiply the distance by 1,66 and then start my way further down if i see no problems. Also keep in mind the plants could stretch a bit.
so in an example:
Actual disctance: 0,5m x 1,66 = 0,83m
or 2ft x 1,66 = 3,32 ft