I think the answer to your question is about 8° smaller at the valve (i.e. comparing to a hydraulic). That has always held true for the ones that I have measured and would guess that the 8° rule-of-thumb to be +/- 2° depending on lobe and lash setting.
IMO, if the 220/220 hydraulic is just a little to small, then the 246/246 mechanical will be more than just a little too big.
If your concern is idle quality, you should maybe compare the advertised durations too. Some hydraulic rollers have a pretty big on-ramp.
I think 8 degrees would get me in the ballpark of where I want to be. I'm not concerned with idle quality. This is my 4WD mountain truck. I drive it on the street but in 4WD low in the mountains it runs out of RPM pretty fast. With the wide ratio transmission I end up losing too much RPM. I want to just stick it in 2nd gear in low range and leave it there. So maybe 3000- 6000 RPM range.
The 220/220 cam pulls well from about 1200 to 4000 I just can't seem to find the sweet spot with the gearing. But what concerns me is it being a complete pig on the street below 3000, especially with the wide ratio transmission. I guess I'll find out.
