Hi
Do you have her marriage certificate to James Boyle? This would give her father's name and occupation, hopefully, so you could pin her down as a child. I notice there are quite a few Elizabeth Johnson's born in the Leeds district. 1855. 1856, 2 x 1859, 1861.
However having said this, if she is not sure of her age then whoever registered the death won't be certain either. The death Jen found is a good possibility. Have you an idea when Richard died? If he was after the date given by Jen then I think he would most likely to have registered the death and you could ask the local registry office to check that before ordering the certificate.
evie