Need to turn plain wholemeal flour into SR wholemeal flour

[Pluvia]

Have said I'll do a carrot cake for someone's birthday tomorrow. Using a tried and trusted recipe (Felicity Cloake) that requires wholemeal self-raising flour. Tried to find it in Tesco and Sainsbury's today and neither has it. I have wholemeal plain flour. How much baking powder do I add per 100g?

I've seen various different recommendations: everything from half a tsp to 1.25 tsps. And how does one calculate a tsp? My teaspoons vary in capacity so much that some are twice the size of others. I could go for a middle-sized one. Level tsp of baking powder? Flattened down firmly or loose?

I'm an experienced cook but I've had issues adding baking powder in the past and am quite nervous of it. Too little and you don't get the rise. Too much and it rises and collapses and tastes weird too. I've got to deliver a decent cake and I'm nervous!

https://www.jamieoliver.com/inspiration/easy-flours-guide/

I usually use 1tsp for every 4oz of flour. I never buy self raising flour and I've never had a problem.

And? I know about the different types of flour.

Thanks! Level tsp? Rounded? Heaped?

I Would go for 1tsp, which is 5g.

@Pluvia. Well, if I'm being fancy, I get my measuring spoons out but, generally, I just use a big standard teaspoon and get a sort of heaped teaspoon full of it. I'm not very scientific but I've never had an issue.

if you look at self-raising section it states

SELF-RAISING FLOUR
This flour is key for baking. Raising agents are mixed with plain flour for the light, airy result we love in cakes, batters and puddings. These raising agents create air bubbles in dough and batters, perfect for fluffy muffins, classic sponge cakes, or American-style pancakes. You can simply sift in two teaspoons of baking powder to each 150g of plain or wholemeal flour if you don’t have self-raising in your cupboard.

1tsp = 5ml

For baking powder, there is around 5g in a tsp.

For baking/cooking a teaspoon is a level 5ml spoon. You can eg use a medicine spoon if you have one lying about

I'd use a heaped teaspoon per 100g, use a middle size one.

I'm getting the feeling this isn't a precise art! Okay. I'll try and get over my fear of baking powder. Thanks all.