As the brewers mentioned, having an initial dough in mash temp of 120F may not be the best. Protein rest temps are based according to the SNR number or the Kolbach number. The numbers indicate level of malt modification.
Dough in for a mash temp of 95F, like the other brewers recommend.
The 1st decoction should be pulled when the mash is at 5.8 pH. Rest briefly at protein rest temp determined by SNR, before being heated to conversion temp. Conversion temp is based on pH of the mash. Higher the pH, higher the conversion temp. The 1st decoction is used to hit protein rest temp in the main mash.
The 2nd decoction is pulled soon as main mash temp is stable. Without knowing the SNR number, waiting 20 minutes before pulling the 2nd decoction may over extend the protein rest if high modified malt is used. The temp needed is 128F to 130ish for protein rest using high modified. The protein rest is skipped in the 2nd decoction, because the main mash is in the protein rest temp. Conversion temp of the 2nd decoction is in the beta temp range. Usually lower than 148F. The pH determines the conversion temp.
When using high modified, mash pH will stick at pH created in the malt during kilning. Most lager malt will be around 5.8 pH, unless brewing water is real crappy. While the 1st decoction is taking place, check pH of the main mash. If it is stuck at 5.8 add sauer malz half way through the decoction time period. The pH will drop to 5 or a little below after completion of the 1st decoction.
Boil both decoctions until the foam is reduced. This will lessen hot break in the boiler, reduces hard starch, makes fly sparging easier and increases hop utilization. Test for conversion in both decoctions at 10 minutes. If conversion isn't evident, add mash liquid. Conversion will occur quickly when the decoction is thinned. Complete conversion may not take place during a 140-148F beta temp in the 2nd decoction. Wait 20 or 30 minutes, then, boil it up. The 170F rest won't be needed if you boil longer than 10 minutes.
Low modified malt is more suitable for decoction method brewing. Better LHBS carry Weyermann floor malt or Crisp Malting's Euro Pils. Using low modified, a brewer relies less on brewing water pH. The malt is enzymatically richer than standard malt.