If you can jump it to the side until you get the distance that is the safest, but these days you can't do that at most tracks. Ask the riders clearing it what gear they are in, how far into the powerband, whether they are steady throttle or on the gas hard for more lift/distance. I short jump doubles first, landing just short of casing it, to get the feel of the takeoff ramp. Find a jump with smooth takeoff, not a kicker, to learn on. Once you are comfortable short jumping it, typically 1 gear higher and on the gas moderately hard will clear it easily. Most 30-45' jumps on typical tracks in my area can be easily cleared in 3rd on a 250. Probably 4th on a 125. You really need to do a lot of flat landing jumps for distance until you are very comfortable with that. That way you can overjump a double at first. Power on landing, body forward, for smoother landing. Body forward on takeoff, legs snug against bike. Be familiar with panic rev and mid air weight shifts and mild whips before trying any serious doubles. There are a lot of unsafe doubles out there, be careful.