Difficulty having the required erection

Hello,
I am struggling to have the required erection in order to Jelk.
I have no erection problems, when I’m a sexual encounter, or even by myself when masturbating.
However, going to jelk is not in line with the above, therefore: no erection whatsoever.
And thusly I face: when I have an erection, it is because I am not going to Jelk, and when I am going to Jelk I am not getting an erection.
How do you guys achieve this forced erection? Seems too hard to me, my erection is more like an involuntary reaction, something that happens because I am excited, it is not like moving my arm, when I can force it to do what I want.
And btw, it gets even worse when it is taught that we need to have “60-80% erection. I am struggling to get a simple erection. And when I do get it, it is in a 100% one. I simply cannot understand having a a “60% erection”, either I get an erection or not, meaning either it is 100% or a limp 0%.

How to solve this? Currently I am jelking after masturbating and cumming. Just wait a little for my penis to soften a little. The problem with this is that I have a limited time to jelk before it goes soft.