Well, scales, obviously. It's way too easy to guesstimate at portions and guesses get bigger and bigger and bigger, don't they? Good neither for pocket nor for for weight. My scales get a better work out than I do (not that that's saying much!).
This how I work out the price of portions.
Price of item (P) divided by weight of item (W) to get the cost per gram multiplied by however many grams you are having (N). P/W x N.
That's where my calculator comes in handy. It's one thing to divide when the pack is 1k,100g or some other nice, easy amount, but not so easy when the label says 78g, for example.
I have a calculator in the kitchen and anther one by the PC. Just very basic, cheap things that do the job beautifully.
For basics, I have the cost per portion on a sticker on the packet, e.g. rice, pasta, porridge, sugar, etc, but always I check a new packet. It's shocking how rapidly prices are going up now and what was correct last week might not be this week so when it has been several months since you needed that item, you have to recalculate.
Very helpful gadgets indeed!