Breastfeeding, pumping and formula??

I wanted to breastfeed but also pump so if my partner wanted to feed him he could. It would also be useful when out and about when breastfeeding could be harder. (Eg not being able to stop ect so having to push the pushchair and feed)

However I'm worried my supply won't be big enough to produce enough milk for breastfeeding and expressing (or if I accidentally forget to express sometimes) so I was thinking of breastfeeding and formula feeding.

Does anyone have any advice on this? Do I need to use the same ratio of breast/formula milk? Would I need to feed the formula at the same time every day/ same amount? Obviously if I'm Justin the house I'd want to just breastfeed so don't want to have to give formula every single day?

I'm totally unaware of what I need to do (due in a few weeks)