JV - early exit multiples/penalty

Hi All,
Hoping you could help me with a "technical" JV question, given my experience is much more biased to the IPO world; I'm a bit out of my depth here
My company is currently negotiating a JV with a 3rd party.As part of the agreement, we have in place an option to buy their share of the JV on Year 5 under a certain EBITDA multiple (dependent on growth rates achieved).

Now, we would like to add an “early buyout option” for us to buyout their share of the JV in year 3. However, the counterparty still wants the EBITDA multiple then to be based on Year 5 projected growth.

My question is whether there is a standard way in which to apply a “penalty” to the valuation should we exit/buyout the JV earlier

• Do you apply a discount to the multiples because now are reliant on 2years worth of projections and exiting 2 years earlier?
• Or do you use the year 5 multiples and just discount the value back to Year 3 at an agreed-upon WACC?

Any thoughts/guidance as to the more "standard" way to reflect an early valuation "penalty" is much appreciated.
Many thanks in advance!