A correlation between shear strength and the clay condition (very soft to hard) you find in table 6 p. 24 of downloadable publication

500PLUS REIDBAR Design Manual

I quote the undrained shear strengths Cu...

Very soft Exudes between fingers when squeezed in fist 5 kPa

Soft Easily penetrated by thumb 9 kPa

Medium Difficult to penetrate strength with thumb 20 kPa

Firm Easily indented with thumb nail 37 kPa

Stiff Readily indented with thumb nail 75 kPa

Hard Difficult to indent with thumb nail 150 kPa

Notation Errata: The written values are Su our undrained shear sthrength as stated, not Cu as wrongly referred.

Greetings 3trax

I realize you are seeking an estimate of the soil Su from Att limits, but if all you require is an estimate why not use your SPT blow count values.  Based on my experience, this conversion method has accuracy of approximately
+/- 40%.

A more accurate method, but much more involved method would require looking at the Su/Overburden stress ratio.  This method involves looking at the Platicity index and a correlation chart published by Skempton.  (I think other people have also published a similar chart?)  

Some other items which you will need are; the depth and overburden stress at the sample depth and an understanding of how the Su/Over stress ratio correlate for N.Consolidated and Over consolidated soils.

Best of luck
Coneboy   

There is great paper title "Shear Strength Correlation for Geotechnical Engineering" by J.M. Duncan, R.C. Horz and T.L Yang from the Virginia Tech. Department of Civil Engineering.  This paper 100-page paper is very useful, but at the same time can be dangerous.  Dangerous in that final design should always be based on actual test results.  However, for preliminary analysis it’s a good start.

from Page 62-63 for undrained strength they suggest:
Su/sigma'vo = 0.45 * liquid limit (as decimal)

Su/sigma'vo = 0.11 + 0.0038 * plasticity index (in percent)

from page 89 for residual angle of friction:
for Ip = 10, residual angle of friction is 27 degrees
for Ip = 20, residual angle of friction is 22 degrees
for Ip = 30, residual angle of friction is 17 degrees
for Ip = 40, residual angle of friction is 14.5 degrees

Hope this helps !
DRT

Also you can find information about Shear Strenght Correlations in the paper title:

Manual of Estimating Soil Properties for Foundation Design by Kulhawy and Mayne
Electric Power Research Institute, Palo Alto California.