I have been trying pomelo marmalade recently, using the peel. We eat the fruit  ( I think you need a serrated knife to help remove the membranes off each segment  to enjoy it) and save the large pieces of peel.

I pre-cook the peels by putting them in cold water, bring up to a boil, cook for about 5 mins and repeat twice more, with a view to reducing the bitterness. I save the last lot of water, and allow the peel to cool (or do the next stage in rubber gloves) and cut off a good section of the pith and discard that. Then cut up the softened peel as you wish, and continue making marmalade as usual.

I use sugar in the ratio of 2:1 to the weight of fruit. So for 500g of fruit I would use 1kg sugar and 1L water. I have followed Fiona's advice from jam making and taste the marmalade before it comes to a rolling boil and only add all the sugar if it needs it.

The first time I made a 3 fruit marmalade using 350 gm pomelo rind, 295 g of lemon and orange peel saved from other uses. I softened all the peels in the reserved pomelo water made up to 1.2 L then added 1.2 kg of sugar. It took about 20 mins of boiling before setting, but set very firmly in the jar.

This made 4 jars of strongly flavoured marmalade. Ideal for those who like bitter flavours.  So far, family and friends have enjoyed it too!