I think a preferred metric ideally is not to have that 30% test. When I looked there were 20 EU countries with similar wage subsidies in both amount and structure to Canada's. It's a mix of those that have a sales or gross revenue decline and those that have kept it really clean and have zero additional requirements that way. The British program is an 80% wage subsidy for all employers regardless of their size and circumstances. That to me is the cleanest way of doing it. Larger firms with more sophisticated access to tax and accounting advice may be able to manage that fairly well. I might suggest that you look at a threshold for smaller companies, perhaps with a couple of hundred employees or fewer, that could become exempted from the 30% test. That is one way this could be accomplished to ensure there isn't a risk or fear on the part of business owners from using it.
Remember, there are some firms that have major sales declines, but for many it's a very uneven way of understanding the impact this is having. There are tons of businesses that won't have a problem with the 30% test because their sales have fallen below the floor because they have zero sales, but for many it's going to be very imprecise and risky to go ahead and end up not meeting those challenges. Not only are you facing the penalties from government, but you are facing the potential bankruptcy of your firm, because if you have gone ahead and been paying your employees full wages and you don't get the subsidy, your business is done.