Let me see if I can somehow answer the question, but I need to go a bit broader for this, so you will not get a straightforward answer, I am sorry!
I want to first say: What will be in this post is my opinion formed on the research that I have done. I do not claim that this is the way to go, I do not claim to have done fully exhaustive research on this.
I want to start with the antacid question in general. I personally do not like the usage of proton pump inhibitors for myself, such as Tagamet and Rantidine. Proton pump inhibitors will, as the name says, inhibit the proton pumps in the stomach. They lead to a
long-lasting reduction of of stomach acid production, not stomach acid. Taking this type of drug half an hour/ an hour before taking SN will not do much to the HCl content in the stomach. The HCl already in the stomach will not be affected, merely the production of new HCl, which is great if you want to not have a high HCl amount in your stomach after your next meal, but before SN it will not lead to the desired effect. The time to act for this type of drug is quite slow and it requires as far as I understand it, at least one more meal before your HCl concentration is lowered (the meal will use up the already existing HCl in your stomach, the proton pump inhibitor will result in a lower concentration of new HCl produced, so only at this point your HCl concentration in your stomach is decreased). So this is the perfect drug to use if you do the 48 h regimen of meto and you do the proton pump inhibitor in parallel. But for immediate, short acting drugs that reduce HCl that has already been produced and is present in your stomach, proton pump inhibitors are not your choice of drug.
I will personally go with milk of magnesia for that exact reason. It will act very fast, on the HCl that is present in the stomach, not the one that will be produced over the next x hours. From what I have read, it is difficult to assess when to take it as usually it is not done in advance, it is more of a 'Oh, I have heartburn now, let's take it' kind of drug. People suffering from heartburn have reported they felt better within 15 - 20 minutes after ingestion, which is fast. This is also what the pharmacist told me, when I asked her when my stomach would stop to burn.. (You guessed it, I've never had heartburn in my life, but I had to ask to get the information needed). So personally, I will take it 20 minutes before the SN.
Now to cover the question of amount.. this is tricky. Because you want to take enough to neutralize a large portion of the HCl, but you don't want to trigger the apparently very strong laxative function that milk of magnesia also has. Apparently, 'For constipation problems, it may take 30 minutes to 6 hours to produce a
bowel movement.' (Source:
https://www.webmd.com/drugs/2/drug-326/milk-of-magnesia-oral/details) I have done a test with 5 ml for myself. I have low stomach acid to begin with, so I might go for the recommended 10 ml dose or stick with the 5 ml, but I will not double it as some have suggested. I think dying is hard enough as it is, I don't need to have the runs at the same time.
One reason I will also stay with milk of magnesia is because of this tidbit I found (same source as above): high magnesium can result in a slow/ weakend heartbeat. Seeing as SN can lead to tachycardia, this might be a way to (minimally, mind you) counteract that.
This is a huge amount of text, but I don't want to just give you a 'Go with xx minutes before', because I don't know the exact answer, I am a guniea pig for this as much as the next guy on the forum. There is no recipe for death by SN, so I thought if I give you my reasoning you can either accept or reject it based on the information given, rather than believe in a magical number I pulled out of my *ss.
My God, I'm sorry, I write too long posts.