Have I screwed up my RMS installation?
 

I thought I had figured a clever way to install my new Rear Main Seal to the requisite 13mm below the crankshaft flange... well about 2/3 clever. It is now at about 14 or 15mm. Is this acceptable or do I have to pull it and install (another) new one?

Pull it, throw it away, and install a new one to the correct depth.

JFP,
Do you use any lubricant between the seal OD and the engine?

Absolutely not, this PTFE seal MUST be installed meticulously clean down to no finger prints, and dry, and NEVER use any sealant around the outside of the seal.

To get the seal seated to the 13mm spec. suggest you Search for some of the diy tools used.Renntech has some. It is very tricky to do and each practice session costs you yet another seal ! Suggest you review the seal installation tool options you find here before you invest or try them.Most involve pvc pipe fittings.The trick is how to fit a precise 'stop' to limit how deep you drive the seal.