To my knowledge it is simmilar build to the european version (esv4020 by the label, EurocenterV50). On those machines there is a fine sprout gear like piece on the spindle near to the v-belt section. That piece is being watched by the rotary encoder (magnetic device) to provide accurate spindle position info. The spindle reference point should be a hole or added mass to the encoder ring or a missing teeth.the spindle orientation offset is stored In a parameter.
The proper distance between the rotary encoder and the encoder ring is crucial so as the runout of the encoder ring. Setting inproper distance between the ring and the encoder cause an error message on orientation command because of the abnormal feedback from the encoder . So dont panic on first attempt failure, just use some spacers while thightenig down the encoder. As far as i can recall our machine neded about 0.2mm gap. Smart move to write down the working gap size to some obvios place in some relevant part in the manual like the spindle assembly/parts list section so it goes faster next time ,You have to move the encoder.
If the spindle renew guys do proper job, they have marked the spindle and the encoder ring to each other so they were reassembled in the same angular position as it was before disassembly for renewal so it does not needs any adjustment for spindle orientation offset. Same thing with the runout of the ring cus the encoder is a bit#h I must say.
For the belt tension, You really need to feel that, besides take a look at the power draw of the spindle on the full rpm spectrum and listen for abnormal sound from the motor or the sindle. After all these loook fine, take the biggest torque requirement tool used to be used on that machine and try it out, like a big roughing facemill, if that pass i guess your done, try not to overthighten the belts because a bounch of other problems can arise from that.
Hope I was helpful