Just been going through similar with CurlyJones. You will get closer and closer to the 200 but it will be painfully slow. I would do as recommended to him that you make the new water around TDS 100 when you do a water change, so after the next that should reduce the TDS to about 225 afterwards. The important thing though is to add the new water gradually, I know a dripper isn't practical with your large tank so maybe add 1 gallon every hour or something (only necessary as you are changing the TDS by a noticeable amount). Keep water change water to the TDS 100 until you get to the figure you want to get too!
You should ALWAYS top up between water changes using pure RO water with no mineralisers!
You should be able to use the spreadsheet I attached to CurlyJones even though it is in litres but it should still work the same as it is the proportion that is more important I think anyway. I attach a copy changed for 60 gallons and 10% water as it makes calculating the new water TDS requirement much easier?
TDS Calculator.xlsx
It won't be 100% though unless you know the correct figures to input, ie my 35L tank actually has 28L of water, the size of the tank would only be 35L if it were filled to the very top and had nothing else in it! Hope that makes sense?
Hope you haven't had any more shrimp deaths.
Simon