How to use multiple check boxes to update a number variable?

My goal is to have a number variable (“X”) that ranges from 0 to 1. I will get my Output by multiplying my Input by that X variable. I want to modify X depending on if any one of several checkboxes are ticked.

I have made it work so far with only one checkbox at a time. I have an output which has a long IF formula (If Box I is true, then Output is (Input-(Input * 0.25)), else if Box II is true, Output is (Input-(Input * 0.2)) and so on with 0.15, 0.1, and 0.05. I realise this doesn’t work if I want it to factor in another active checkbox at the same time.

Question: how can I update a number variable based on several checkboxes?

Ideally this would be without using a button and my output would automatically update. Any help would much appreciated! Thank you : )

Edit: so far my solution is to have a long formula for the output. I have three sets of checkboxes and the user will select one box in each set. I have a formula for the first set, then minus the formula for the second set, thirdly minus the formula for the third set of checkboxes. I hope that makes sense.