The eluting strength is classified with the eluotropic series. In this series the solvents are arranged in order of their relative abilities to effect elution or respectively the order of solvents runs in the order of their polarity.
Low polarity n-hexane ⇒toluene ⇒chloroform ⇒dichloromethane ⇒acetone ⇒dimethyl sulphoxide ⇒acetonitrile ⇒isopropanol/n-propanol ⇒ ethanol ⇒methanol ⇒water ⇒salts and buffers High polarity
If you have a mixture of acetonitrile/water 50:50 (v/v) and want to switch to methanol, you need about 20% more methanol to get the same eluting strength. You also have to consider the possible interaction between the different molecules or functional groups of your sample, mobile phase and stationary phase.