Do you use a consistent time for making contact (proof) prints of your negatives? If so, then you should be able to look at the contact sheet and know if the enlargement will print close to a standard time, need more time or less. After you have made a few enlargements from your negatives, you will know the base exposure time for each enlargement size. If a proof print of a 35mm negative looks good and it normally requires a 12 second exposure for enlargement, you know to do test stops with 12 seconds as the mid point.