I'm sorry if I'm not seeing something obvious here, but can you tell me why do you even have any FB's or loops or anything.. like Lare and janner_10 said...
Since all your tags are scattered around the program and you have to link them manually anyway, why don't you link them directly to the needed tags...
Even if you did what you wanted with for..next I just don't see it cleaner then a simple
"ModbusComms".Reg4000[0]:="Analogue_In_DB".FIT101_Scaled;
"ModbusComms".Reg4000[1]:="Analogue_In_DB".FIT102_Scaled;
etc...
Clean, with visible indexes at first glance, much more readable..