- Although I've tried, I cannot find the "formal" equivalent of the term but my proposal is “reduced payout calculation method" based on the definitions I've found.

- As the bottom websites in Japanese explain, this method of payment of damages/compensation is calculated based upon the amount of damages, minus the deductible, multiplied by the reduced rate/ratio/percentage agreed upon in the insurance contract. Depending on the percentage chosen, the "payout" is reduced.

- Also, I prefer the word “method” for “方式” since it is most frequently used in reference to different “methods” of payment (or payouts) in insurance plans. You’ll get many hits for “payment method” with regards to insurance: Please see websites below:

http://ejje.weblio.jp/content/損害保険
損害保険 <損害>
nonlife [property] insurance

Here are two great websites explaining your term in detail:
http://www.kaito.co.jp/co_earth/
縮小支払方式
実際に発生した損害額に対し、決められた割合分だけが補償されます。
《お支払い例》
火災保険金額1億円（建物の価額）、地震による損害額3,000万円（実損額）の場合
火災保険金額 1億円
地震支払い縮小率 50％
免責金額（自己負担額） 10万円
お支払い額 （3,000万円－10万円）×50％＝1,495万円
最終的な自己負担額 3,000万円－1,495万円＝1,505万円

http://hokenyougo.lify.jp/word/1/価額
縮小支払方式 しゅくしょうしはらいほうしき
損害保険
【意味】
価額）×縮小割合

Please also see the charts in the website which explain how it works:
http://www.hoken-jishin.com/kigyo/tokuyaku.html
縮小支払方式
■ 保険金の支払方法
　 （損害額　－　免責金額）　×　縮小割合
◎ メリット
　 比較的低廉な保険料
× デメリット
　 損害額が縮小割合まで削減される
■ 契約実態

　 ・はじめて地震保険を導入する企業
　 ・限られた予算で不保検討の企業

http://structuredsettlementinsurance.com/how-to-boost-your-i...
How to Boost Your Insurance Payout on Property Damage Claims
http://www.doityourself.com/stry/insuranceguidehome
Insurance companies use two different methods to “calculate the payout in the event of a catastrophe” which renders the home unlivable. The first is the actual cash value and the other is replacement coverage. The actual cash value provides the actual cost to replace the damaged or destroyed property which comparable new items, less any depreciation.

http://www.e-insuranceonline.net/injury-claims-and-things-yo...
That’s where an insurance company’s or companies payout calculations and computations basis comes in.

http://www.carsdirect.com/car-insurance/how-to-get-the-bigge...
How to Get the Biggest Car Insurance Payout