Think of the distance you move through a turnout, but in feet, or some other unit. For every foot you run along the main axis of the turnout, your diverging route deviates at a certain rate if the train takes the diverging route. For a #4 turnout, the diverging route moves one foot to the left or right for every 4 feet of travel down the main axis. A #8 turnout runs 8 feet down the major axis for every foot of diversion. If you can picture it, a #8 turnout takes twice as long to diverge a single foot, so the frog angle is about half of the frog angle of a #4 turnout. The sharper the diverging angle, the smaller the frog number. In the prototype, except for street cars and some industrial track, it is unusual to see anything much smaller than about a #8. Out on the high speed rails, turnouts that make crossovers run about #25 and on up. Very shallow angle at the frog, and a very loooooonnnng turnout, probably 50-80 feet and even more for speeds in excess of 60 mph.
If you are on a curve and need to diverge, you get a 'curved' turnout. The trick is to get the right one. A Walthers #6 or a Peco #6 (I think Peco calls theirs middle radius or something like that, you'll have to look) CURVED turnout, left or right, is probably what you need. But investigate the twin radii for curved turnouts and see if you can't modify the curve enough to make such a turnout fit.
