Why add sugars step wise instead of just adding up front? What is the benefit of adding in steps? I just made a mead with OG of 1.130 which should get to 20%. It was my first attempt so I hope I did not mess up. Used red star champaign yeast bc that's what I had.
With an OG of 1.130, you're looking at (a max of) 17.6%, assuming it goes to dry at .998 FG. Don't assume it will go beyond that, but be prepared in case it does. Actually, I just looked that yeast up... NOT a good choice for your batch. Since it's tolerance is 13-15% ABV, your batch is also going to finish sweet, and not near 20%. Well, not unless you hit it with another yeast strain, which is not nearly as easy as you might think. You're probably looking at a FG of 1.017-1.033...
As for why to step feed the sugars, it's to give the yeast less stress. Putting yeast into a must with a high OG means it will be stressed from the start. I did that on my initial batches and it took it much longer than it would have if I had step fed it.
I hope you gave your must nutrient, energizer, and plenty of oxygen at the start (jtrux)... Otherwise, you could have an uphill battle on your hands to get it to finish, or finish without taking a couple of extra months. My initial batches were still creeping to 18% after over four months in fermenter. At six to seven months, they were at 18% and done. They did finish sweet, which helps to balance the 18% ABV.
For a 21% (or anything close to that) batch, you're best off NOT making the must with that high of an OG. You need to formulate the recipe by the final ABV level (and OG that it would take to get there), use the same level of water, and less honey/sugars. That way, when you add the additional honey later, the volume actually hits the target to get the ABV level.
BTW, ALWAYS research your yeast before selecting it for a batch. This is a prime example of NOT using the correct yeast for a batch. To get to 20% you need to use something like Wyeast Eau de Vie (goes to 21%), or a strain that you have a chance of 'pushing' beyond it's normal range. There are stories of people pushing yeast like Lalvin EC-1118 or K1-V1116 beyond 18%, closer to 20%, but those are experienced mazers that did everything correctly to get there.