The short answer is: Think about the lady's weight changes, and time yours to hers. You have some leeway in this respect, since her weight change on step 2 is just fractionally more gradual than yours (she has space to cover while you don't), but this aim alone will get you in the ballpark.
From the book perspective, you turn 3/8 *between* 1 and 2, so you should be facing approximately diagonal center at the moment you transfer weight from LF to RF.
The heel turn, whether danced by man or lady, has roughly the same sequence of actions, and similar mechanics in many respects (the most notable exception being rise & fall, but we'll save that for another discussion): First you step backward, then draw the front foot to meet the back foot, closing initially without weight while turning on the heel of the receiving foot (e.g. if the step is back on LF, turn is on the left heel). At the end of the heel turn, weight is transferred to the ball of the opposite foot, rising to toe, and continuing to turn if the figure calls for it -- which the Impetus does.
If you have trouble turning on the heel, you're not alone -- It is probably the most common problem I see amongst men dancing the Closed Impetus. The heel is a very small target, and so it is easily missed. Most novice dancers are content simply to find balance on a foot; To balance over one specific part of the foot takes more skill. This skill, as any, is sharpened through repetition of the correct action. So make sure, first and foremost, that when you practice either alone or with a partner, your aim is for your heel, not just your foot. If you miss, you can then analyze what caused you to miss, and fix it.
The most common cause of a missed heel is early rotation. You can curve gently as you move the body weight away from the right foot, but once the left foot is placed, the feet must track and weight must move in a straight line until the weight arrives fully over the heel*. In particular, the left side of the body must move straight to the heel of the left foot. It's very tempting to rotate prematurely, which has a nasty tendency to deflect the weight short of hitting the heel.
You're much more likely to hit a target that you're aiming for, and awareness is always the first step toward fixing your aim. If you're not aware that the weight has to move all the way to the heel before the turn begins, or that the closing foot must not take weight until the end of the turn, or that early rotation can deviate your aim, you're just shooting aimlessly in the dark. Understand those points, and you have something to aim for. The rest is just a matter of repetition.
As an exercise to help sharpen the skill, practice stepping back on the left foot, lifting the right foot and placing it next to the left ankle an inch or two off the floor. This will ensure that your weight is on one foot only. Pause momentarily before turning to check that the weight is actually over the heel, then turn. For this exercise, the actual amount of turn is irrelevant so long as the action is correct. Do not sacrifice precision in an attempt to achieve a certain amount of turn, or you've missed the purpose of the exercise.
I hope this all makes sense. As always, these things are much more easily demonstrated than written about. Let us all know how you're doing with it after a week or two, and remember: Never be ashamed of your Impetus!
Regards,
Jonathan
============
*Some people teach that the upper body continues to turn to a certain limited degree during this moment of straight movement, but it cannot do so in such a way as to affect the linear movement of the left side of the body to the left heel, nor can it affect the direct tracking of the feet. If it does, you'll miss the heel. On expert dancers, this slight twisting action has a beautiful smoothing effect on the heel turn. But if you're still working out your habit of early rotation, you're better off focusing on straighter tracking and full weight delivery to the heel.