Spider01, there is no way of predicting this, the only way to solve your issue is through practice, practice, practice. Just get fast and accurate with calculating everything. It's really not difficult but does take plenty of repetition to get fast and accurate. If you're doing PPL nav's then you won't have too many legs to worry about either.
My suggestion is if you know what your route is a week in advance, simply print out a bunch of templates with the route and every day get the area forecast and do the calcs, you'll find after a few navs and doing a couple of dozen mock flight and fuel plans you'll become quite quick at the whole thing.