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It depends if the floor is flat but sloping.
If so, you could get away with option 1. fitting the plinth to floor height and ignore the gap above or pack with something appropriate to your skill /ocd.If the floor is noticeably lumpy (or a better option from above) scribing the bottom edge to suit would be best. Option 3
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It's pretty normal to scribe them to the floor in kitchens I'm making. Same with skirting boards.
Place the plinth on the floor and mark the undulations, cut that with a jigsaw. Now it's scribed to the floor cut a straight line to match the line of the bottom of the cupboards. Allow a little wiggle room.I like to add the special plastic strip under the plinth material as it stops them sucking up moisture and covers any ragged edges from the jigsaw.
Other retailers are available.
Kitchen plinths: mine are nominally 80mm high (Ikea) but in some places the floor is uneven and so the height is larger - I've wound out the height adjusters on the cabinet legs to accommodate.
How to deal with this for the plinths? I'm using solid timber. Option 1: use 80mm timber, leave a gap at the top in some places. Option 2, gap at the bottom. Option 3, cut a taller piece of timber to suit.
I am strongly leaning towards option 1. Is this the "normal" way of things?