The concept is the "HBU" aka Home Brewing Unit for hops measurements.
The first one in the recipe you linked is 1.5ozs of EKG (4.5%).
One ounce of 4.5% hops has 4.5 HBUs in it.
If you had a 2.25% hop, like maybe some Saaz, you would need two ounces (2*2.25) to get 4.5HBUs in the kettle.
So you got 1.5ozs of 4.5%, 1.5ozs*4.5%= 6.75HBUs
What is in your freezer? 4.0% East Kent Goldings, OK. 6.75HBU / 4.0, you need 1.6875 ounces of the 4.0% to make 6.75HBU in the kettle.
In practice, my hop scale rounds to the nearest tenth of an ounce, I would make that 1.7 ounces of the 4% and find something else to worry about.
I kinda wish more people would post their hops additions in HBU, that way half the math is done already.
If the recipe says 7HBU ofChinook at (30) all you have to do is look at your bag of 13.5% Chinook and know you need about (just a little less than) half an ounce.
There is going to be more than one way to handle flameout hops and they are each correct.
What I would do on my system is toss in the Irish Moss at (15). At (0) I would turn off the burner, pour in the 5.6 pounds of ME, whisk that up good, then put in the hops, stir the heck out of the pot. Leave it covered for ten minutes or so and then move on.
I had five minute hops in my PM brew yesterday. At (0) I put in about three pounds of ME, whisked, and steeped about 10 minutes. Then I poured through a grain bag to get the hops out, back in the kettle, and then I dropped in my sanitized wort chiller. After it was cold I stirred it up real good for whirlpool, went to bed and then got up at 0400 to siphon off the wort. I pitched at like 0422 this morning.
Some brewers (with bigger kettles than me) will leave the (0) hops in while the whirlpool settles and siphon off that after the (0) hops have been in the wort for several hours.
The equipment you have is your system and the way you use it is yourprocess.
Part of the experience for me is finding ways to improve my beer, which usually means more equipment, which introduces another new learning curve for the process. It's a never ending cycle, but a plesant one.
Don't sweat the hops too hard. If you are close it is going to be beer.