Optimizing of Bushings clearance.

That is a major research / engineering / consultation subject involving big bucks. You got to be kidding!

Assuming your plain (hydrodynamic) bearings are of cylindrical shape the clearance choice can be related to journal diameter and journal surface speed. The makers of bearings using tables or formulas for their determinations/recommendations.

However, the bearing clearance often is used as a "design feature" for manipulations of rotor-dynamic machine features.
Most common the bearing clearance is expressed as a "relative bearing clearance" - related to journal diameter (multiplied by 1000 ==> means: clearance in 1/1000 of the journal diameter). This relative clearance is named with the greek letter PSI.

It is a good practise to machine the bearing diameter in a "uniform hole" tolerance ISO H7 or H6. The clearance respected with the machining of the shaft journal. The German DIN standard # 31698 provided clearances and shaft deviations in a table sorted by classes of PSI. For horizontal machines the PSI classes 1.32, 1.6, and 1.9 are most common. The smaller 0.8 and 1.12 classes merely are used for very large drives and/or low speed applications like large ship propulsion drives.

Following general relationships can be expected:

- the bigger the journal diameter the smaller the relative clearance PSI

- the higer the journal surface speed the larger the relative clearance PSI

Down and dirty is to take the bearing clearance chart for diameter and speed, then read it for "recommended clearance. Use engineering judgment to make allowance for bearing area, pressure, cleanliness of operating environment, temperature, etc., and adjust clearance as needed for specific application.

The Question, as posed, is indeterminate. There are way too many variables to assess given the complete lack of definitive information. This suggests the Questioner lacks comprehension of the complexity of the subject and would benefit from a thorough background discussion as a prelude to obtaining sufficient information to begin a fruitful dialog.