where to enter estimated income when K1 has NOT yet arrived?
Your only option would be to enter the k-1 amounts as though you had received them. That's not a great idea unless the k-1s don't vary from year to year. K-1s are difficult enough to fill out when you actually have the forms. It's too late now, though, to request an extension and pay estimated tax with the extension. You may now underestimate your k-1 income and you could still owe a penalty and interest. You may also end up having to amend your return once you have the k-1s but you have left yourself with no other option. Hopefully, the amounts don't vary much.