Bonjour Claude
With many ascos as the spores develop in the asci they grow until mature so taking measurements of immature spores will give you inaccurate figures. The added problem with Scutellinia is that some of the spores shrink in size as the outer layer of the spore coalesces to become the ornamentation(up to 6µ shorter according to Schumacher). With some species if the spores have almost matured they will take up the stain even whilst still in the asci and using that information and the details of the hairs you can come to a reasonable conclusion. Another proviso given by Schumacher is that imature spores can show more or less ornamentation than ejected spores.
So you can give identification a go with your immature specimens but the chances of success are lower.
Mal

Hello Claude,

you can check, if there's any leaf, piece of wood etc. over the hymenium of some apothecia - the sporeprint might be on the lower side of it. I saw this once with S. cejpii, one apothecium was dried with a fragment of a leaf on it - after removing it, it was white from the lower side with lots of nice mature spores.
Also, if the spores in asci near the margin are not completely mature, you might try to take a small sample from the centre of the hymenium. Often there's higher percentage of mature spores there than near the margin. I got this advice from another mycologist, who's working a lot with Scutellinia and I can confirm that it worked for me too. But I used it only with a few collections, because taking a sample even with thin and sharp injection needle makes further damage around the puncture and about a quarter of the spores in the sample were crushed. Maybe would I have cut the whole apothecium in half and then made a thin section from the middle, it would be less damaging, but I didn't try that yet. 
With my own collections I always try to get sporeprint on aluminium foil (a piece of dark paper would be perhaps better?), which I later pack together with the exsiccate. 

Kind regards,
Viktorie 

Hello Mal,

I take good note of everything you just explained to me and I will try, knowing that my chances are rather slim.
Best
Claude

Hello Viktorie,

The idea of looking around the collection on a debris is very interesting and I will look carefully. For my part, I cut the Scutellinia in four, then make a thin cut from the middle to the margin; Finally, I will see my future collections. For spore print, I usually put on a transparent plastic sheet protector, which I then cover. Eight times out of ten, I get a nice spore; which is essential for measurements and observation of ornaments.

Best regards,
Claude