I haven't flown the Midwest version, but as I recall its got airfoli shaped tail surfaces vs flat rounds stabs.With my .60 size one (72 inch wingspan), I have two ways I do a "tumble". The first is I will roll inverted (looks better to me) then corner both sticks to the left as you do (full throttle, full down, full left aileron, full left rudder). It will generally tumbe 2-3 times and fall into a knife edge spin when the energy drops.
A second way is I climb at a 45 degree angle, and basically do an upright snap (full up elevator, left rudder, some or full left aileron). I hold this and the plane snaps two or three times violently until its almost stalled and its generally nose up (at full throttle), at this point I put in full down and it typically does a couple more tumbles and often ends up in an inverted falling leaf type stall.
They tumble better with an aft CG, but move it carefulley as I once watched a RC bipe of mine spin down to the ground as I had a too far aft CG and it wouldn't stop.
Try different timings of when you put in the controls etc and eventually you will find what works best with that model.
Dave Hamblin
(formerly DaveH on old site!)