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The discretionary income to be used depends on the level of your comfort and not even about the market environment and If you say buying on a Lump sum is worth while buying on the down side then to me you are describing buying the dip and not buying with the lump sum, because talking about buying with the lump sum has nothing to do with the market conditions but rather an investor decision to buy immediately with the available Lump sum amount irrespective of the market conditions,
The amount of Discretionary income should be best related to your amount of income per week or monthly no as long as your basic responsibilities do not grow up to occupy the extra increase in basic income. This is where cashflow management comes in. It is a general knowledge that humans responsibility virtually grows to meet every increase in a person's income. Let's say that a person is willing to buy aggressively when there's increased pay, some get distracted by getting a new car loan or changing their apartments to suit their assumed new standards.
How about maintaining your status prior to your new income with little or no rise in your standards of living. As a bitcoin investor who has an accumulation target, it is very important to focus on your mission and instead of getting oneself involved in new car loans even when you have a car you used before. You can decide to assume that the increment in your income never occurred by channelling all or part of the new pay into your bitcoin. This would help you to accumulate more bitcoin recently and allow you take part in the bitcoin future prospects.