Okay....OP got luck in that I am motivated, mobile, in a good mood and the excrement seems to not hitting the fan today ( though a possible hurricane appears to be lurking now).

I have a Jaeger's 5 inch lens set. F5 ish. So, I got the lens out in the sun. I measured the focal length of the set at 23.5 inches. I measured the lens separation (center of element to center of element) at about 0.5 inches. Measured the positive element at 11.5 inch focal length. Focal lengths probably within an inch and maybe more like 0.5 inches. Rather that do any algebra I just used "Astroboys" online calculator for DIY eyepieces to solve for the focal length of the negative lens.

I came up with -21.5 give or take. That is about a minus 540mm. Not what the OP needs.

However, going with the assumption that a stereotypical achromat (using bog standard glass and design..which I am fairly sure describes most of Jaegers mainstream products). I did the following calculations.

What if you had a 4 inch f8 Jaegers lens set? So, lets scale that....32/23.5 would give you a factor of 1.36.

Or if you had a 4 inch f10 Jaegers lens set? 40/23.5 would give you a factor of 1.7 or so.

1.36 times 540 is about 730mm.

1.7 times 540 is about 920mm.

So, at first glance it sounds like a 4 F8 Jaegers lens might work...and an f10 is right in the ballpark.

Of course more careful measurements would be needed....

Good news is that an f8 lens set is for sale right now here on CN.

Or another way to look at it. There is a good chance that any run of the mill bog standard achromat that would work needs to be in the 800 to 1000mm focal length for the negative lens to be in the ball park. Are there 80 to 90 mm achros that have those kind of focal lengths? (I have an old Meade 90/f8.8/800 but THAT lens set is glued together...though one supposes they could be separated given the right know how).