Personally, I don't like the 'do ikkyo then transition to nikkyo'-approach, unless one is training how to recover from a poorly performed ikkyo by changing it to nikkyo. Focus on the nikkyo from the start.
Secondly, I was taught nikkyo ura with an entry and setup that's different from ikkyo. So for reference (not suggesting here you replace what you're being taught with what I describe here
): small entering step to the back side of uke, one hand grabs forearm of uke other cuts on the wrist; cutting motion is down while performing small tenkan (for the correct position, as Abasan said: centerlines!); keep control of uke with hand on forearm, other hand goes round uke's wrist to take the nikkyo grip while putting uke's hand/wrist against your shoulder.
p.s.: The technique description above was written with a gyaku hanmi shomen uchi attack in mind.
If Ukes attack is right Migi Gyaku Hanmi Shomen , I would do the following :
1. Blend with Ukes , stretch out my left hand , followed very closely with right hand and secure very firmly Ukes wrist at near zenith of Ukes attacking hand.If you do this too late Ukes shomen is too powerful on the downward motion.You must catch the wrist early.
2.Adopt a Gyaku Hamni [left leg forward ] position, ensuring that you are of the line of Ukes attack.
3. Maintaining hold of Ukes wrist with own left hand , utilise your right hand and cut uke down [use heavy hand ].This breaks Ukes posture, the rest is simply placing Ukes wrist in hollow of your own left clavicle/shoulder region, and then applying pressure to the wrist/forearm.Needless to say both hand and footwork , body positioning must be done in unison with good timing.
Your method would appear similar to my own.