As
@Sadfield has said, the reason for your poor head retention may lie elsewhere but nevertheless here are some ideas:
1. I recommend reading at least this short section of Strange Steve's guide to water treatment as it outlines exactly what you're trying to do -
this link then scroll down to the section
Putting it all together -
Brewing a stout with low alkalinity water
2. Treat your water with Campden tablets if you aren't already to remove chlorine/chloramine.
3. In Brewer's Friend you can create and save water profiles - it's worth doing this for both your source water at home, and your target water profile. For the latter I have assumed we're using
Strange Steve's stout profile as a target. In the BF water calculator scroll down to Source Water and click Configure to create & save water profiles (or
this link might work)
4. Create your recipe as normal in BF. You need this to get the grist in your water calculator (it's a pain to enter manually in the calculator). In the Water Chemistry section of your recipe click the green Edit button under Water Calculator - the water calculator will open in a new window.
5. In the water calculator under Source Water select your
home water profile created at step 3 advice. Scroll down to Target Water Selection and select the
target water profile also created at step 3.
6. Still in the Target Water Selection section, review the target water profile values down, along with the Actual and Delta values shown. Note that in the Delta section you probably have lots of reds indicating you're some way off your target profile. When we start adding minerals below you'll need to scroll back up to these Deltas and use a bit of trial and error to get your salt additions right.
7. Now scroll down to Salt Additions. Have a play adding different amounts of Baking Soda to get the HCO3 Delta close to zero. You don't need to be too precise, getting it within +/- 10ppm is good enough so whole, half of maybe quarter gram amounts are close enough depending on the accuracy of your scales.
8. Repeat step 7, this time adding gypsum to reduce the SO4- ("sulphate") Delta close to zero.
9. Repeat step 7 again adding Calcium Chloride (dihydrate) to reduce the Cl- ("chloride") Delta close to zero.
10. Note that gypsum and calcium chloride both contain calcium. So as you add these you're also bringing your calcium levels up to roughly where you want to be.
11. When I made my calculation I was able to get HCO3, SO4 and Cl very close to the target. Calcium (Ca) was a touch low, about 10ppm off the target at about 140ppm instead of 150ppm but I'd be happy enough with this. I'm also happy with the mash pH.
N.B. In my calculation the baking soda addition results in sodium ions (Na) being raised to about 50ppm. I simply don't know if this is ok or not so you'll need to research, and potentially find another salt to add to get your HCO3 up.
12. With regard to the target profile - I think it was John Palmer I've heard say water additions are like seasoning, like adding salt & pepper etc while you're cooking. It is true that you might want to alter the target depending if you're making sweet stout, dry stout, American stout (more hoppy), and also depending on personal taste.
13. Last step, click Save/Update on your water calc! Close or and go back to your recipe. Under the Water Chemistry section of the recipe, under Water Calculator click Update Recipe to bring through the results of your calc. Scroll up a little to Other Ingredients and you should see your additions listed.
14. I can share my recipe and water calc with you, but try it for yourself and see how you get on.