I did a child support calculation and it looks like if I used that for 50/50 split; I would owe her $600/month. I know she will ask for this. Is it reasonable that I pay it now; since I think I would be legally obligated to pay it if it were a true divorce?
Yes, if you're confident you calculated it correctly. But I would ONLY pay it either:
a) Directly (such as guy something yourself for the child(en);
or
b) Reimburse her if/when she provides you with RECEIPTS. I wouldn't give her unconditional cash payments.