And then I've sucessfully used 269 on PE 850 DI single MAF. 555 DI single maf 90-94 style uses 415. As for the latency, I use .81ms. Jwt uses .83ms for 555's.
390 falls right under the K-value of 415 for 90-94 555's so it makes sense to start here. I've tried the formula from devin Z and its not very accurate. The formula gets you close enough to give you something to work with but its trial and error to get it on the dot.