I'd like to suggest a feature where we can enter a custom amount of credits to partially speed up an upgrade.
IE: The game already calculates the amount of crystals required to instantly complete something. It then converts that into credits according to what the current crystals -> credits ratio is, and then finds out what % we've entered as compared to that. We spend those credits and the progress moves by that %.
This is for those times when you're near your credit cap and don't want to overshoot. EG: You want a 4 Mil RS Scanner upgrade and have 3.95 Mil. Your credit cap is 4.05 Mil. There's an additional 200k waiting for you in BLS rewards but if you win you'll overshoot and lose some of the rewards. So you want to spend a smaller amount and not lose any rewards.