A campden tablet in the vessel and then put the water on top will deal with any chlorine issues
As for Murphys, I don't like the one size fits all approach they advocate ... Recommendations are OK, but only apply to a limited range of 'English' Beers. Also the recommendations haven't changed in 25 years, despite the increase in knowledge and techniques at our fingertips.They are biased to the products they sell, fair enough, but what if your water is not standard. you can treat a wider range of waters better using individual acids (Hydrochloric and sulphuric rather than AMS/CRS), and individual salts (calcium chloride and calcium sulphate rather than DWB / DLS). If you like a sulphate forward liquor, and that's what the Burton brewers had, then fine, if you want to go chloride forward, like Sam Smiths (Landlord anyone) then you are stuffed.
For one vessel brewing, add your campden tablet and start filling, after you have around 5L in it, add 2/3 of your acid, once you have 15L in there, add your salts. calcium sulphate is soluble!!!
it is a myth (almost) that it is hard to dissolve!!! the solubility limit for calcium sulphate/gypsum is around 2.3g/l, so if you are trying to dissolve 20g in 1 litre of water you are going to fail, but dissolve the same amount in 15 to 20 litres and it happens with no issues. If you already have a high level of calcium in your liquor then you may still experience issues, but you probably won't be trying to add a huge amount of calcium anyway (or shouldn't). Still it's a good practice to add your gypsum first, and then to add any calcium chloride afterwards. Once you have all the water in the vessel measure your alkalinity again, and add the final amount of acid to bring the alkalinity where you want it to be.
The advice to add it to the mash is somewhat out of date, as there is the belief that it required the 'acids' released from the malt to help it dissolve ... it dissolves quite happily, I know a commercial brewery that adds the Acids/salts to the HLT while bringing it back up to temp first thing in the morning as they are recircing it ... then mash and sparge. It's the first 15 to 20 minutes that the mash pH is established properly, if you don't have sufficient salts dissolved at that time the pH will be all over the place
Just my thoughts