Our ratio is currently
Taxable = 20%
Roth IRA, Roth 401k, & HSA (tax free) = 15%
SEP/401k (tax deferred) = 65%
Like the OP mentioned later, this question also very important for getting the ACA subsidy (worth about $11K a year). We managed to get the subsidy last year as we had some inheritance to live off, so didn't have to sell too much from taxable (we are semi-retired).
For many years we were in the 25% bracket, so we stuff'd our extra income into tax deferred to get us into the 15% and free cap gains-land. That was before the ACA, and the O-MAGI really complicates things.
In general I've been selling our Taxable account and moving it into our Roth 401k. Mostly selling the "low hanging fruit" (stuff with little cap gains).
Now we have to raise funds to buy a car this fall, and it's going to be a headache to figure out how to keep our MAGI low enough for the ACA subsidy refund if we sell a lot of stocks from Taxable. Will have to weight into dipping into our Roth IRAs and HSA, or see about financing half of the purchase price. But really, first world problems and all that; feel fortunate that we are where we are.