It isn't going to be a problem for you topping it off. I process over 20 rolls per kit in mine and I always have water carry over from my rinse after developer so sometimes I have to dump a little out once my 1L bottles are over-flowing.

My problem is usually dwindling developer from leakage. I have no problem adding 100ml to it if I need to top up. But only once. I tried a second top-up once and had sudden death.