The rule of thumb is that you do not want to overdistend the bladder and cause nerve damage. There is a formula for peds the age and weight of the pt. in kg calculated into cc's. If the bladder feels distended or if the pt. complains of feeling the need to void but can not..then cath and totally empty the bladder. We use a bladder scan ultasound machine that helps us keep up with the amount. If the bladder scan shows greater than 500 on a child 10yrs and up it's time to cath.If we have a baseline measurement that the pt. can hold more then we will let them go a little longer but not much over 550. Rule of thumb for us is if you have to cath 3 times the 3rd cath replace the foley. I do not understand not emptying the baldder fully. There is only vagal response if you bear down. Bladder spasms come from irritating the bladder trigone area. Leving urine in the bladder sounds like a set up for another cath and a UTI. Hope this helps.