Investment rules don't apply to newbies alone it cut across to all investors both old and new, it's all about investing rightly within ones financial capacity with which they can be able to survive with what's left after removing their investments funds from the total amount they earn in either weekly or monthly.
I think it should be the other way round buddy, discretionary income is always what is left after catering for your expenses, which is what you need to decide the percentage of it you invest into bitcoin that would determine your level of aggressiveness. If you do not consider your living expenses first before investing into bitcoin, then there is a possibility that you might invest more than what should be your discretionary income and have to prey on your portfolio for survival at some point.
Another thing which is my personal opinion is that the amount with which we invest with monthly doesn't necessarily have to be static. As in, you can decide to invest $10 this month and probably put in $20 the next month thereabout depending on how things turned out to be for you financially within the previous month. Because there's a way you can spend frugally and do have some extra bucks left into the new month.
It is good that you have a regular accumulation amount that you wouldn't want to go below it. Also it is possible you may have lesser expenses and more discretionary income at some periods and you wish to increase your aggressiveness, you are still on track since you are drawing closer your accumulation target at a faster pace.
Again at some point, you may have relatively more expenses and invest lower than supposed, it is very necessary that you make up for it in preceding periods. The goal is never to leave your accumulation amounts to chance, else you might be overly whimpy in it which is not the best. Having an accumulation amount helps you remain focused and dedicated in your accumulation journey.