I was going to buy a meter, posted on here and after the discussion I decided to can it.
If you change your water at 3-4 months, you really don't need a TDS strip or meter. Water in my neck of the woods is cheap and it has been said in the past 'water is the best chemical you can put into the tub'.
There is a formula in the Taylor book that tells when to change the water, even that can be wrong.