Well, you can usually turn those last two in the rotation around. Specially if you have the talents to refresh SnD with Envenom.
Open with Garotte then Slice and Dice, the 1 or 2 combo-points you'll have at that point will be enough for the next part. Then 5 points worth mutilate into an envenom to max out your SnD duration. Themiller is right that you would like to wait for your energy to re-pool here for a bit. The bleed on Garotte should still be trickling away happily, giving you enough time to get more points and put up a 5-point Rupture when the bleed from Garotte drops off. Now, don't panic if an envenom or rupture falls off. You can only string a perfect combo against a practice dummy. Wait for 4 points at least, wait for energy to return; specially wait if you're on interrupt-duty to try and keep around 1/3rd to 1/2 of your energy bar in reserve. Your dps output won't suffer too much for it.
Never finish under 4 points. If you're assigned to go dps adds during a bossfight, always try and leave a Rupture ticking on the boss. Also switch to Backstab during the execution phase of a boss' hp (36% and below) once you have the appropriate talents/glyphs.
I may be overly redundant, but I should be asleep right now, not posting on forums