When can I do a Roth Conversion after a reverse rollover?

I want to do a backdoor Roth IRA by contributing after-tax dollars to traditional IRA and then immediately converting it to Roth IRA. However I have a traditional IRA account with some money in it, and this makes any Roth conversion a taxable event.

 

I plan on doing a complete reverse rollover from traditional IRA to my current employer 401K. Once that's done I will start backdoor Roth.  My question is how soon can I do Roth conversions after a reverse rollover in order to avoid the pro-rata tax rule.