Hi,
You can't restrict a coupon's shipping percentage to the shipping cost of only one warehouse.
Either the coupon applies, and it applies to the total shipping cost, or it doesn't apply.
You can configure the shipping methods so that the shipping is free for one warehouse and not for other warehouses.
But then, it doesn't involve the use of a coupon. In that case, you can use the restriction settings of your shipping methods to achieve this. Maybe that could work for you ?
Now, if you need to require the user to take an action (like enter a coupon code), to get the discount, this won't be possible. In that case, the only solution would be to develop a custom plugin to recalculate the coupon value on the fly based on your specific requirements.