Sleeeeeeves.

[Puzzler]

I am attempting to make a toile of the Chelsea Blouse (https://inhousepatterns.com/products/chelsea), View B with the V-neck. As you can see from the photo of the Chelsea, the front underarm seam is slanted forward so already the back armhole is longer. I come along and make my round back adjustments and a forward shoulder adjustment. I altered the sleeve cap to give me more room at the back armhole seam. That's all good. The problem is I have far too much front sleeve cap width. See my photo of the armhole edge without the side seam sewn. Note how drastically smaller the front armhole is compared to the back. What on earth can I do? I don't want a sleeveless blouse.

[DementedFairy]

I am attempting to make a toile of the Chelsea Blouse (https://inhousepatterns.com/products/chelsea), View B with the V-neck. As you can see from the photo of the Chelsea, the front underarm seam is slanted forward so already the back armhole is longer. I come along and make my round back adjustments and a forward shoulder adjustment. I altered the sleeve cap to give me more room at the back armhole seam. That's all good. The problem is I have far too much front sleeve cap width. See my photo of the armhole edge without the side seam sewn. Note how drastically smaller the front armhole is compared to the back. What on earth can I do? I don't want a sleeveless blouse.

I would suggest you start by measuring the bodice carefully along the stitching line, and the same for the sleeve, so you can more accurately compare the amount you need to lose.  It could just be a case of moving the 'top' of the sleeve towards the back to compensate for your adjustments

[Ohsewsimple]

Have you adjusted the sleeve for the forward shoulder adjustment? 
https://inhousepatterns.com/blogs/news/the-forward-shoulder-adjustment-two-ways
If you can’t sort it why not email her.  I would imagine she’d be happy to help.

[Puzzler]

DF - I've measured and have moved the shoulder point to a position where the back bodice matches up well with the back sleeve cap seam but that's created more of a problem in front. OSS - I have adjusted the sleeve to take into account the FSA. When measured, I end up with 4cm or about 1 5/8inch excess. Will try some clipping to see how much that helps. Otherwise I will write to Alexandra. I get her weekly emails and watched several of her fitting tutes.

[Ohsewsimple]

Not sure how the FSA would result in excess if you’ve adjusted to take that into account. 
The round back adjustment shouldn’t make any difference.  I see the side seams are further forward but that should be accounted for in the sleeves anyway.   Mmmmmm.  Just puzzling this in my head. 
I think I’d go back to the original pattern and as DF said, measure the seam line from underarm to shoulder on back and front separately and then the same on the sleeve head to see what there is on that. 

[Puzzler]

I managed to get the sleeve in only just. However when I tried the top on the armhole seam wasn’t right. I will have to take one step back and re-fit the armhole. Plenty of fabric. It’s only calico at this stage.

I decided I am getting nowhere fast with toiles so have dug out a twice used Butterick pattern for a top with set in sleeves. I am going to make that up in proper fabric before I lose the will for dressmaking. I could then try to replicate this armhole/sleeve on the Chelsea blouse.