First prepare the fruit:
Using a vegetable peeler, remove only colored part of peel from grapefruit; chop and set aside. Peel and discard white part of peel from fruit. Section out the pulp, save any juice with the fruit and remove any seeds. Clean and chop the strawberries into large 1" chunks (they will break-up more in cooking).
Prepare jars and lids:
Prepare 8 1-pint mason jars; wash in very hot water or in dishwasher. Place lid-caps (portion with the rubber seal) in a pan of water and bring to boil. Reduce heat and keep hot until ready to use.
Prepping grapefruit and peels:
Place grapefruit peels, 2 1/2 cups water and 1/8 teaspoon baking soda in saucepan. Bring to boil. Reduce heat; cover and simmer 20 minutes, stirring only occasionally. Add grapefruit and juice; simmer 10 minutes. Remove from heat and combine with cut strawberries until you have 5 1/2 cups fruit with juice. Ratio of strawberries to grapefruit doesn't matter. Almost 1:1. Squeeze the juice of one lemon into mixture.
Prepare the pectin and sugar:
Measure 4 cups sugar into a medium bowl. (Do not reduce sugar). Mix 1/4 cup sugar from measured amount and 1 box Sure-Jell For Less or No Sugar Needed Fruit Pectin in small bowl.
Cooking the fruit for jam:
Place large stock pot (the deeper, the better) on stove. Add the 5 1/2 cups fruit to the pot. Stir in the pectin with the 1/4 cup sugar into fruit mixture and add 1 teaspoon butter to reduce foaming, if desired.
Bring mixture to full rolling boil (a boil that doesn't stop bubbling when stirred) on high heat, stirring constantly.
Stir in remaining sugar quickly. Return to full rolling boil and boil exactly 1 minute, stirring constantly. Remove from heat.
Quickly, place jars on protected surface on counter. With a large funnel to help prevent spilling, fill all mason jars with the boiling fruit mixture. Fill to 1/4" to 1/8" from top of lid. With wet rag or paper towel, wipe off any jam that may have spilled onto top of lid where sill will touch.
Place hot ring lids onto jars and then add lid bands. Tighten until you feel resistance and then give it just a bit more tightening so they are tight, but not overly tightened.
Immediately turn jars upside down. Remain that way for 5 full minutes, then revert jars back to correct standing position. Jars should seal over next 30-40 minutes or so.
If any jars don't seal, place in fridge and use within month. Otherwise, sealed jars are good for up to 12 months.