Learn to do something which is more measurable, or figure out ways to measure some particular aspect of value you can bring. In back-end software, it will most likely be around performance or security. Doing a security audit then proposing remediation... if the company has already been hacked, they (should) understand the value rather quickly. If not... it would be a harder sell.
Performance? Would be hard, I think, without being a brand and people seeking you out. Speeding up an ecommerce site can definitely have measurable impact on bottom line but... if a company is big enough where that would have meaningful impact, they probably already have a team of people to navigate through to convince them of your value/skill/ability.