A few factual discrepancies in there:
We hold 73 seats in the European Parliament out of 751 total, that's 9.7% far higher than the 1/28th you say we have, the only time we are 1/28 is in our right to veto.
Similarly our net contributions after our negotiated rebate were 11,341M€ from a total budget of 142,496M€ so only 7.9%.
As for the EU regulations, we currently write most of them with the chairs of over half the CEN working groups being representatives of UK industry. If we leave we will lose that voice but will still need to abide by the regulations if we wish to trade with the EU, at present there is nothing bar common sense stopping any company from offering goods or services to CE standards within the EU and offering a different specification elsewhere.
If we exit any deal we cut will be very unlikely to involve the rebate but is likely to see us paying in the same as we do now so we'd be closer to your 10% figure. We'd have no seats, no vote, no hand in setting the regulations and no veto any more whilst the chances are that we would have to maintain freedom of movement with all 28 EU member states to even cut a deal.
I fail to see how you think we will be better off.