The first thing you need to rule out, is whether the cause of your early bleeding is related to too little estrogen.
2 pumps is really not much estrogen. If you don't take enough estrogen, it won't be able to sustain the lining of your uterus - which will break down more frequently and shed. The type of bleeding which is usually caused by this is quite light.
So I would first and foremost do a blood test on day 3 of your cycle to check your E levels. (This is hopefully when your own E is low, so you will see what you're getting from the HRT.) You want your E to be at least around 450pmol. Some people are okay with lower, but the bare bare bare minimum is 270pmol and that is pretty low....
Assuming your E is over 450pmol (by the way, it is very unlikely that with 2 pumps of gel, you are over 450pmol...), then you need to work out the utrogestan as follows:
You need at least 11 days of utrogestan per cycle to protect yourself. If you bleed consistently around day 20, then you need to count back from there to determine when to start it. Which takes you to day 10 as your start date. So you would start utrogestan on day 10.
Ta da.
But I reckon you are probably not on enough estrogen because very few women are....