Is there a rule of thumb for 'safe' margin percentage?

Ignoring borrow costs, margin is leverage and it is a double edged sword. If you use 25% margin then you have the potential to make 1.25 times or lose 1.25 times as fast. Margin simply magnifies the P&L.

The best rule is to avoid margin until you are an experienced investor/trader and you understand position sizing, you understand and practice disciplined risk management and most importantly of all, you have an edge which provides decent gains for some period of time. There's no one size fits all answer for how much margin to use. The specific amount is an opinion.

I'd suggest that you consider options as a possible substitute for modest leverage. Buy high delta deep in-the-money two year call LEAPs that have a reasonable implied volatility instead of your margined equity positions and put the remaining cash in an income vehicle. The time decay will be nominal during the first year and there are some other advantages as well. I've written a number of answers here (search for "Stock Replacement Strategy" or google it).

Another interesting approach would be to use an Option Repair Strategy for leveraging a new position. Rather than dive into detail, here's an example. AAPL closed at $126.65. A March $127.50/$140 repair could be put on for a slight credit. Per 100 shares, between $127.50 and $140, you would make $2 for every $1 that AAPL rose, with a maximum gain of $25 in 3 months (nearly 20% in 3 months). If AAPL does not rise, the options expire worthless and it's as if you just owned 100 shares. That's free leverage for the first 12-1/2 points up. If this idea interests you, google "Option Repair Strategy" for details.

The drawback of the Repair is that you cap you gains. If $25 of upside isn't enough, you can go out further in time for more potential upside. For June you could have 40 points of gain on a 20 point move. 50 points of gain on a 25 point move for September, and so on. All at no cost. My preference is shorter term Repairs like March but that's another discussion.

The cost of this upside leverage is a willingness to cap the gain and you increase your loss potential by ZERO, something that is not true for the increased margin that you are suggesting.

The same warning applies for these option strategies. Do not attempt them until your fully understand them.