Starter Explanation Please

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Hi Everyone

i want to understadn a few thing about the starters. If I have a 2 weeks old starter, 100% hydration, whole wheat an water. I try keeping 15gms all the time after discarding. my question is if I feed it with 30gms water and 30gms flours now I have 75gms of starter? if a recipe calls for 50gms starter. I can take from the top of the jar 10gms and mix it with 20gms flour and 20gms water and wait until it doubles and then use it or is there something im doing wrong?

I also need to understand when using it, is It better to mix it in the water before combining the rest of the ingredients?  or Is better to put all the ingredients together and then mix in the starter?

Thanks

Hanry

"if I feed it with 30gms water and 30gms flours now I have 75gms of starter?"

Once it matures, i.e. doubles in size and takes on the acidic aroma, yes

"if a recipe calls for 50gms starter. I can take from the top of the jar 10gms and mix it with 20gms flour and 20gms water and wait until it doubles and then use it"

Yes, it's just usually easier to just scoop in 50g from the jar