The tap size is actually controlled via the "Tap Positions" field. So with a tap range of 0.9 ~ 1.1, you have a range of 0.2. With each tap step of 0.00606, the number of tap positions is 0.2 / 0.00606 which is 33.
The current tap setting will either be "Winding 1 Ratio (pu)" or "Winding 2 Ratio (pu)" depending on your control settings. You can also use the "xfrdat" routine from the psspy API to programatically retrieve the tap ratio for two winding transformers.
Here is a series of batch commands to add a 2-winding zero-impedance transformer between a hypothetical bus 1 and bus 2, as generated by PSSE. The winding 2 ratio is one tap off the nominal ratio, at 1.00606.
BAT_TWO_WINDING_DATA_6,1,2,'1',1,1,1,0,0,0,33,0,1,0,0,1,0,1,1,1,0.0, 0.0001, 100.0, 1.0,0.0,0.0, 1.0,0.0, 1.0, 1.0, 1.0, 1.0,0.0,0.0, 1.1, 0.9, 1.1, 0.9,0.0,0.0,0.0,0.0,0.0,0.0,0.0,0.0,0.0,0.0,0.0,0.0,0.0,0.0,0.0," ",
BAT_TWO_WINDING_CHNG_6,1,2,'1',,,,,,,,,,,,,1,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,;
BAT_TWO_WINDING_CHNG_6,1,2,'1',,,,,,,,,,2,,,,,,,,,,,,, 1.00606,,,,,,,,,, 1.03, 0.97,,,,,,,,,,,,,,,,,,;