How to you do the calculation to figure out material weight in the hose ?

Well the first step is to figure the area of a circle. The circle being the inside diameter of the hose. There are some online calculators that make it easy and keep you from needing to pull out your hair.

The area for the ID of a 1 1/8" hose is .99 inches

The area for the ID of a 1 1/4" hose is 1.23"

Next you figure the area for the length of the hose. a 100' hose with 12" in a foot comes out to 1200 inches

Then you multiply the area for the diameter times the area of the length. .99 x 1200 =1188 1.23 x 1200 = 1476

Now what that is telling you is that your 1 1/8" hose will have 1188 cu inches of material in it and the larger hose will have 1476

A cubic inch of water weighs 0.036127 pounds (google will tell you that)

So we take .036127 x 1188 and you will have basically 43 pounds of material in the hose

We take .036127 x 1476 and that tells you that you will have a bit over 53 pounds of material in your hose with the 1 1/4"

Of course you add the weight of the hose to that.

A Finn unit can put out just a bit over the 70 PSI that the hose is rated for however like many things there is a margin of error built into that. Typically about 50%. This is common with a lot of things. I know the company that we looked at as a hose supplier tested their hose which is rated about the same 70 PSI and went up over 135 PSI before they really had any problems so the PSI rating may be a touch low but should not cause any problems. I have lots of Finn guys using our hose which has a similar rating and have not heard of any problems.

There are different manufacturers of hose and the quality can vary. For example quite a long time ago we tried some Kuriama "Tiger Flex" hose which looked identical to what we use and did not have good luck with it at all and we tried some Goodyear which was good but really stiff. I think you will be fine trying that hose.

One other thing to keep in mind in the 1 1/4 vs 1 1/8 decision is that fittings for 1 1/4" might be far easier to find.