I'm a St. Louis CPA so I have the tax experience, but thus far I have accumulated very few real estate investments. The real estate experts might have a better take on this, and you might want to get a more local/regional opinion as I think this can change by location, but for what it's worth here's my two cents.
We use a flat 10-15% allocation to the land for real estate clients at our firm, unless there is a strong reason to choose another percentage/dollar amount. I've never been part of an IRS audit where this allocation was in question, so I'm unsure if it would hold up. Obviously as you mentioned the more you allocate to building the more depreciation you can deduct. A high building % is aggressive, and the IRS might not like it.
Also, when you sell the real estate, every dollar you depreciate (which saves you ordinary tax rates) decreases your costs basis, therefore it increases your capital gain (which is paid at the lower cap gain rates, but subject to depreciation recapture(25%)). This is ultimately a good thing in the long run, but it can create a large tax bill at the end of the venture if you simply sell.