Sale was grossly underpaid before his raise: Porcello is not being overpaid, right now. Many felt Sale would make more than $30M per year, and with the deferred money, his $29M a year is really less than that.
I do not think Porcello will make $20M+. I could be wrong, and comparing him to the older Keuchel was not a good comp on my part. I could see him making $16-17M tops, but I would not offer him more than $15M x 3 or $14M x 4 years, and I'm a big Porcello supporter. I loved the extension when many did not, because he was in prime the whole time.
I'd start with an offer of $50M/4. Second offer: $42M/3. If he says no, maybe I'd go as high as I said above, or maybe I just offer him a QO and be fine with either way it ends up.