Even though my tds meter is potentially not accurate can I use it as a guide ie itís reading 200, my shrimp seem happy, so just use that number without calibration ? Then I can reconstitute my ro water to tds of 200. Or is it better to reconstitute to a gh reading?
This is exactly how a TDS meter should be used - to compare one sample to the next in full knowledge that the actual number is meaningless except for how it differs between samples!
To remineralise your RO water, first choose your product and understand that different products will give you very different TDS's for the same GH/KH. Equilibrium is very "dirty" for example - a high TDS for a given GH because it contains lots of other stuff that is of benefit to a planted tank (but not necessary for low TDS loving shrimp). SaltyShrimp GH+ on the other hand is very clean as it is almost pure GH with very little else.
I would very much recommend the SaltyShrimp products. SS GH+ does not increase KH at all (just pure GH) so is good if your tank has an active substrate and 0 KH. SS GH/KH+ on the other hand contains both GH and KH (surprise!) and will give you roughly 0.5 KH per 1 GH (e.g. 6GH and 3KH). n What's your KH in your current tank?
My preferred way to remineralise is by weight. Pick up a small set of accurate (preferably to nearest 0.01g) scales, not expensive. Instructions on SS minerals give you the required weight per volume of water to add, so with the scales it's easy. Then measure the resulting GH/KH with liquid drop tests and adjust the weight of minerals added to get you to your desired value. When you know what weight of minerals to add to your water change bucket of RO water to is super easy - just weigh out the same amount every time! Fast, simple, accurate.
You can then use your TDS meter to compare your mixed RO water with your tank water, and as a sanity check to make sure that you have added the correct amount of minerals each time.
Rather than weight, you could measure by volume (e.g. find a small Starbucks coffee spoon and work out how many spoonfuls you need to get your desired GH/KH). Not quite so accurate as weight as it depends upon how level the spoonfuls are, how much you squish the minerals in, etc. But another option maybe.
Note that SS products don't use TDS measurements, they use Electrical Conductivity "EC". EC is near enough 2x the TDS (so 200 TDS = 400 EC).