There is no accepted debt-to-risk ratio that makes it reasonable. It depends on your own risk tolerance.
Yep. Like they say with investing "you can either eat well or sleep well!"
The discipline of behavioral finance suggests that the measure of risk is both personal (emotional tolerance for risk) and dependent on personal wealth. For example, while investing 200K for a 50/50 chance of immediately getting 600K in return, is statistically a good bet to take (with an "expected" payoff of $300K). However a person with a lot of assets can afford to make the investment/bet (and perhaps "should"), but the person with few assets can't (if it may cost them their house, relationships, family harmony, the peace of mind) and probably should not.
Also, the discipline of economics suggests that the marginal utility (pleasure) of incremental assets decline. (i.e. the improvement in quality of life of having a 6-bedroom home vs. a 5 bedroom home, is much smaller than the difference between having a 1 bedroom and 2 bedroom home). The same is true of income, the incremental pleasure derived from making $250K vs $150K, is relatively modest compared to the incremental pleasure derived from making $150K vs. $50K. Point being the "financial/mathematics return" may overstate the "utility of that return", when doing a cost vs. benefit analysis.