1x ff sv550 122

A mess. Svbony don't do a 1x field flattener for the 122 so...

aperture ratio   spacing 

80       f6      81,2
80       f7      80,2
102      f7      75,3
115      f7      73,0
125      f7,8    69,7
130      f7      69,7
140      f6,5    68,0
155      f8      57,0 

That's the spacing from the ff itself. NOT from the spacer or m63 to m48 adapter.

sv550 122 f7

flattener fl     spacing from m48

1.0x     854mm        ?

0.8x     683mm       55mm

making sense of it
Examining the ffs, they are in three parts, with the only variable being the length of the centre spacer, here in red.

the TS/Sky Rover 1.0x field flattener
the outer sections are common to all with the centre section providing variable backspace 

TS say...

Ideal para

• Refractores a partir de 700 mm de distancia focal y telescopios RC a partir de 10" de apertura utilizando la distancia de trabajo de 55 mm con el adaptador de transición y el tubo de extensión.
• Refractores con una distancia focal entre 600 mm y 700 mm: la distancia de trabajo se alarga 5 mm.
• Refractores con una distancia focal de entre 500 mm y 600 mm: la distancia de trabajo se alarga 10 mm.

Este aplanador no es adecuado para refractores con una distancia focal aún más corta.

e.g. for a 122mm f7 and that seems to be somewhere between the 102-f7  and the 130-f7 at 854mm
That's a spacing difference of between 75.3 and 69.7 = 5.6mm (~6mm)

therefore the spacing for 122mm should be around 51mm from the m48 of the ff.

made up for testing as follows:

extension tube        20
extension tube         8

m48 to m42 extension   6

touptek backfocus     17.5

                                     TOTAL 51.5mm

But as they say in all good astro forums, starting point only.

Note also the 50mm m63 extension tube without which the focuser needs to be unnervingly far unwound...

...and the high tech focuser counterbalance.