Of course, you can take Bryan Litz's figures and compute a correlation between average G1 and G7 values. Bryan calculates both values based on exactly the same recorded velocity changes throughout the flight at the same fixed waypoints over the same overall distance, and both values are averaged between 3,000 and 1,500 fps. However, look at his figures for the variances within the average.
Taking the .308 185gn Berger LRBT Juggernaut as an example, the G1 values by speed band run from 0.605 down to 0.505, a reduction of 16.5%, whilst the G7 values cover 0.280 to 0.287, a mere 2.5% variation between lowest and highest. Moreover, that small variation isn't directly related to speed (as MV reduces, they run: 0.287, 0.285, 0.280, 0,281, 0.284) whereas the G1's values are. Since in the case of a bullet speed is not only related to muzzle velocity, but to distance, any direct mathematical correlation may range from being roughly correct to very inaccurate for any single segment of the flight, even if the average works out a x% of the other form's average.
So, if we knew at exactly which distance Bryan measures his bullet speeds over, yes it wouldn't matter if you used G1 or G7 form as long as we shoot over the same distance and also employ exactly the same MV as Bryan used when testing that bullet. If you fired the Juggernaut from a .30 BR at say 2,000 fps and then from a RUM or Lazzeroni super-magnum at 3,500 fps, averaged one pair of readings over 300 yards, and the other over 1,300 yards, you'd find the G1 to G7 ratio was quite different.
The other factor that makes so many bullet manufacturers' G1 values suspect is that we simply don't know what the MV was and what the distance was. I'm not so sure about now, but certainly a few years ago many manufacturers' G1 values weren't based on any testing at all. A computer model was used - feed the bullet shape and dimensions in and it produced a 'calculated' (ie estimated) BC, or more likely a range of BCs based on bullet speed of which no doubt the highest was quoted.