I have a virus indigo and when the tempo is set to 144bpm the delay time jumps from 415.1 to 420.2 as well on mine. Why don't you use the delay clock instead? In case you don't know how( I don't have the virus a but I think its the same) go under effects and move till you are at a section called delay clock and turn it on. Specify if you want the delay to be clocked to every 1/4 or whatever. I hope that fixes your problem
