Normally you take the range of your instrument, e.g. 0 to 100 is a range of 100 and divide that by your analogue range, in this case 1000, which would give you 0.1degC per point of analogue.
You can then multiply this by the actual analogue reading, e.g. 200 x 0.1 = 20degC. This is OK so long as you instrument base reading is 0, if it is different then you need to add the base value. For example if you read 50-150degC, an analogue reading of 200 would equate to 150-50 divided by 1000-0 multiplied by 200 plus your base of 50 = 70.
What hardware are you using as there is a particular module for the M221 range, and in that module you configure your instrument type and range and it will give you the correct reading. To me it sounds like you are connecting a PT100 direct to the analogue input which won't work.