Full length resize, but just back off the de-cap pin ensuring it's concentric in the die.
i.e. leave the rod slightly loose while driving the case into the die, then, as you withdraw the case from the die, as soon as you feel the expander ball enter the case neck, keeping the pressure on the ram, tighten the locknut for the decap rod. Decap rod & die should now be concentric.

If you have a headspace guage it's a very helpful tool. I use a Hornady for my MSch 1903 rounds.
They don't show a cartridge specific collar for 6.5x54, so I use the C375 collar (6.5x55).

I have brand new Norma brass, so using the guage it measures a pretty consistent 1.699" mid shoulder to case base (head) No primer in situ.
From my rifle, the fired case headspace dimension using the Hornady guage, is 1.705"
This measurement is taken after removal of the fired primer.
I use a Lee Universal decap die before taking measurements.

This I regard as my 'fired case headspace dimension' so I set my F/L resizing die to resize to that dimension -1.705".
In doing this, I also set the decap rod ensuring it 'dead centre' so that when resizing, I don't have much case neck/body run out & usually achieve +/- .0005"

Personally, I firmly believe working to fired case headspace dimension is easily close enough and F/L resizing to that dimension will extend case life almost as far as neck sizing.
Add further extension to case life if you anneal.

As a further note, to help take up 'slop' in the coarse thread of the die, I use a rubber 'O' ring under the lock nut of the die. This does two things, it allows the die to flex up/down tilt very microslightly, keeping the cartridge true as it's being sized, while also allowing 'pinching' the die up or down to achieve exact headspace dimension. Works a treat!

(It is possible to measure the rifle headspace without go/no-go guages, but it's rather fiddly, so you need patience, but it works.
Rifle headspace is usually a little greater than fired case headspace, perhaps +.003" +/-, so, taking a new brass case, insert into the chamber and pack with thin sheaves of cut tinfoil or rizzla paper, one at a time.

Test the bolt closure on the case until the bolt does not lock down smoothly. Measure the case with foil in place & that's the no-go dimension.

Repeat this process until you reach a point where the bolt closes smoothly and locks down easily and measure. That dimension will be pretty damn close to an accurate rifle headspace measurement).