Wear ring clearance depends on how much leakage flow you can afford without degrading pump head. Stepanoff,A.J.(1957), "Centrifugal and Axial Flow Pumps" 2nd Edition shows in Fig. 10.4 on page 186, leakage in percent for 4 speeds at several clearances and grooved/plain, open/closed end geometries all for a 4.125 diameter seal. For a plain open-ended seal about 6-7% leakage occurs at 4 mils/inch  clearance ratio. At 9 mils/inch leakage loss is 18.7 to 20% at speeds from 1400 to 2500 RPM. If you can find loss factor C, data for a particular seal geometry you can calculate leakage flow from QL=CA*SQRT (2gH) where A is area and H is head. For labyrinth seals, a single tooth loss factor can be multiplied by tooth number minus 1.0. If high running speeds are involved, rotordynamic bearing properties of the seal clearance may become important and more controlling than leakage flowrate especially if impellers are substantially overhung from the lowest radial bearing. Sometimes, noise/vibration control may influence seal clearance selection to avoid exciting highly resonanant boundary structures like thin, bell-like impeller shrouds.

Thank you Vanstoja for valuable information.

Wear ring clearances are dependent on media, bearing material, wear ring material and the size of the pump.
If the pump is old take into account register clearances.
Your pump supplier should have a chart with standard wear ring / bearing clearances for any size pump, this should include ss on ss clearances which are greater than standard pump constructed clearances.

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