I've been doing sheiko for about 18 months (just to qualify my advice).
The first 2 or 3 cycles will suck. I was not prepared, even though I was doing more volume than sheiko calls for.
Doing 3x5 @ 80% is difficult to say the least... starting out. After a while, 80% of your max will be easy.
I found doing 80% @ 3x5 was just too much twice a week, so I only do it once a week and do something different on the heavy bench day of the cycle.
I changed a lot of it up, added in a few things, and changed most of it from basic benching to CG, military press, DB work, etc to make up for lack of variation. My bench moved up and got a lot stronger w/ the 3x5 @ 80% work.
When people say they do not get good gains in bench, just ignore these people. I say this b/c they people IMO are not creative. I moved a bench press that was stagnant at 270 for 2 years past 300 w/ sheiko. I've just heard the excuse too much. I am no genius. If I can get the routine to work by thinking, then surely you can.
Creativity is a major pro for this routine. You can really change things up (after going through 2 or 3 cycles of course). Its simple and at the same time, complicated if you want it to be.
I have a bad back, so I am not able to do the volume sheiko calls for. Unless you also have a bad back, then I can't give proper advice. I did hit a PR w/ sheiko though.
I do not do the 4th day. Some do, some don't. See what works for you.
Like people say, eat, sleep, stay dedicated.
As well, do back work & ab work. This is a given, but not written out in the routine. Also, really focus on your weaknesses and incorporate exercises to fix them.
Do not do more than 4 major movements in a workout.
Example: Heavy squat, CG Bench, GMs, DB Press
If you do more, it should be high rep work for recovery or to address an injury/weakness. Try to keep the volume down if possible. The routine will beat you up.
Concerning the 1RM, once you get it established, figure out how much you can add each cycle. I add 10 lbs to my squat and 5 lbs to my bench each cycle. Deadlift is another story, but I won't go there. I correct my max to produce my old 80% work weight + 10 lbs.
This works out very well. I only calculated my max once and had to adjust it (lowered it) to make the routine initially work. Once I actually got past an entire cycle, I just added weight and continued.
I can say this routine works for squat perfectly. I increased my max 133 lbs in 18 months while not even gaining more than 15 lbs (more fat than I want due to poor diet, damn college schedule!).
I hope this helps.