H -> IDC4 -> P -> TTT4 -> Repeat
Let's break this down. Both Heavy Thrust and Phlebotomize has a duration of 24 seconds. With a Global Cooldown of 2.4s, 24/2.4 = 10. So this means you can fit 10 GCDs in between each of your Phlebotomzie and Heavy Thrusts.
IDC4 = 4 GCDs
TTT4 = 4 GCDs
P = 1 GCD
That's 1 GCD left over, which can be used to keep a fully buffed Heavy Thrust up. This also means you have no downtime on Heavy Thrust, giving you a full 15% damage boost to your DPS.
The same applies to Phlebotomize:
IDC4 = 4 GCDs
TTT4 = 4 GCDs
H = 1 GCD
This means you will never lose HT, nor clip a Phlebotomize tick at 2.4 GCD.