Ok, I can understand that, I’m just telling you how you can get a precise amount transferred. Right or wrong, there seems very little chance that you will ever be able to specify a precise amount for a debit card top-up, as this was rejected/resisted by hugo a couple of years ago, back when top-up was the primary way to get money into Monzo.
I’m not saying you shouldn’t express your desire for this feature, who knows what the future holds! Also, hopefully in the future, with open banking, Monzo will be able to pull money out of your other bank account directly from within the 
app which should provide the behaviour you’re looking for.