How is using a calculator any different to looking down at a map? Hell you can even hold the calculator up in front of your face if you have to.
I always round things up to make things easier to calculate. For fuel. 100lbs of Jet is around 54L. I round it up to 60L and work that out. Once you start taking bigger orders of fuel such as, for example, 1000lbs. For every 100lbs of fuel, you get 6 extra L. 6 x 10 = 60, 600 - 60 = 540L for 1000lbs. (I use this as an example, obviously if its 54L per 100lbs, for 1000lbs you just need to add an extra 0.)
ROD for glide slope is half your ground speed then add a 0 at the end.
My other options are a) Engage Captain Auto if I am single pilot and use a calculator. Or b) Get my F/o to either fly, or do the calculations.