Here's my work-around. These are the tools I use:
The dividers are home-made (wooden to protect display face.)
The Rapid Plotter is from D/L from Pacific Thunder campaign, by Kim Ronhoff.
and a 12" - 30cm combo scale.
I'm using a Sargo in Dec '43 and both the ppi and A-scope work without rings or compass increment.
On my display the diameter of the ppi scope face measures 5cm. radius = 2.5 cm.
Use ratio & proportion to determine distance.
On the 40,000 ppi screen I measure from the center of the target blip to the center of the ppi screen as 1.6 cm, therefore,
2.5 : 40000 : : 1.6 :
25600
I hold the Rapid Plotter compass against the ppi screen to obtain Bearing.
I record the plot per Ronhoff's instructions, (they come with D/L). Later I check my ppi range with that on the A-Scope to confirm my results.
It works.
Just because SH1 had it doesn't mean that it was WWII authentic. Early radar was not an exact science...it was close enough for Govt work.