The SF-25 has such a low wing loading that it easily tolerates the small reduction in maximum lift which the airbrakes cause. I have done very few flights in it (I'm too tall to fit comfortably in it's cramped cockpit), but from what I remember the airbrakes are not very big, having little effect on minimum speed. My experience, however, is with the SF-25 B, and you linked to the SF-25 C, so I have to assume that my experience is not fully relevant.
I have more experience with real gliders and composite motorgliders, and here my experience is: Start your approach, stabilize the right approach speed, set the airbrakes such that your glide path looks right and do not play around with them too much. Set your approach such that you need medium airbrake extension, so you have margin for corrections in both directions. Normally, I come in a little high so I am not caught by windshear, and when I am sure I will reach the intended touchdown point, I extend the airbrakes fully to land with the shortest distance possible. However, when touching down, make sure you do not fully extend the airbrakes: This gives you more time for stopping the descent. Especially in gliders with very effective flaps and airbrakes (ASW-20 C, ASH-25) I never made a good landing with fully extended airbrakes. Reduce them to half extension at the most when touching down!
In the ASW gliders the airbrakes are linked to the very effective wheel brake, and touching down with fully deployed airbrakes means that the wheel is braked at touchdown. This is another reason to reduce airbrake extension when touching down.
In high-performance gliders it is easy to float too long, so it is very important to control your approach speed and to continue the descent all the way down to the touchdown point. Don't pull too early! The SF-25 will not float as much, but with its low wing it shows a pronounced ground effect, and exercising strict discipline when landing helps a lot to make the landing better.
I could not find a number for the stall speed change due to airbrakes in the SF-25 manual, but I would guess it is at most a few km/h.