Dear Stuart Pure methanol at 25C has an Rf of 1.326 The Rf of a 75% methanol/water solution is about 1.345 The range you need therefore is a full range of Rf 1.325 to 1.345 I have found some neat very high accuracy fibre-optic handhelds that have the required range but alas they are USD 650 each. I will see if I can find a nice Abbe handheld locally in the correct range, and send you a .pdf file of the Rf tables for methanol/water. I purchased one for work recently for $180.00. Lance Smith